Topics

HP Power Sensor wanted


Paul Randall G3NJV
 

Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


Robin Szemeti - G1YFG
 

I've been on the same search, with no good result.  

I have drawn up the HP sensor head on a 0.6mm thick PCB and whilst I am very doubtful I could get the thermistor variant of the head to work, I think I could make a reasonable stab at the diode-detector version of the head.

I may pursue this further once winter is a little closer.

If you find yourself with more offers of power sensor heads than you know what to do with, please let me know.


On Thu, 24 Sep 2020 at 21:25, Paul Randall G3NJV <paulfrandall@...> wrote:
Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


--
Robin Szemeti - G1YFG


Robin Szemeti - G1YFG
 

It seems this is as far as I had got.   The "RF" bits were in 0402 size ...

image.png


On Thu, 24 Sep 2020 at 21:36, Robin Szemeti - G1YFG via groups.io <robin=redpoint.org.uk@groups.io> wrote:
I've been on the same search, with no good result.  

I have drawn up the HP sensor head on a 0.6mm thick PCB and whilst I am very doubtful I could get the thermistor variant of the head to work, I think I could make a reasonable stab at the diode-detector version of the head.

I may pursue this further once winter is a little closer.

If you find yourself with more offers of power sensor heads than you know what to do with, please let me know.

On Thu, 24 Sep 2020 at 21:25, Paul Randall G3NJV <paulfrandall@...> wrote:
Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


--
Robin Szemeti - G1YFG


--
Robin Szemeti - G1YFG


Colin Ranson
 

Paul,

 

I too have a HP 435 power meter minus the measuring head. Believe it or not it came in original British MOD storage packaging, with a date of 1983 ! It’s absolutely pristine.  I am reluctant to power it in case capacitors go pop !   I could of course judicially use a variac but even so I think capacitors will fail after a while.

 

Meanwhile it looks pretty on the test bench shelving.

 

 

Colin de G8LBS.

 

Sent from Mail for Windows 10

 

From: Paul Randall G3NJV
Sent: 24 September 2020 21:25
To: UKMicrowaves@groups.io
Subject: [UKMicrowaves] HP Power Sensor wanted

 

Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV

 


Colin Ranson
 

Robin,

 

See my post re HP435.  Please let us know how you get on, I would love to use mine if only a few times to finish uW projects.

 

 

Regards

 

 

Colin G8LBS.

 

Sent from Mail for Windows 10

 

From: Robin Szemeti - G1YFG
Sent: 24 September 2020 21:36
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] HP Power Sensor wanted

 

I've been on the same search, with no good result.  

 

I have drawn up the HP sensor head on a 0.6mm thick PCB and whilst I am very doubtful I could get the thermistor variant of the head to work, I think I could make a reasonable stab at the diode-detector version of the head.

 

I may pursue this further once winter is a little closer.

 

If you find yourself with more offers of power sensor heads than you know what to do with, please let me know.

 

On Thu, 24 Sep 2020 at 21:25, Paul Randall G3NJV <paulfrandall@...> wrote:

Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


--
Robin Szemeti - G1YFG

 


geoffrey pike
 

Hi,
I have used a AD8317 in a non working HP435B and without changing its original state, just use the meter.
From my notes(i actually have them), its performance at 10 GHz (AD8317 spect to 8GHz) shows it
reads about 20 dB low the real value and fairly good to 8GHz. Scaling on the meter is in my case -5dBm FSD and -30dBm mid scale with 55dB range.
The AD8317 does have a -ve output slope, anyway for about a tenner you have a meter that you can abuse and not worry about the thermistor head
cheers
Geoff
GI0GDP

On Thursday, 24 September 2020, 21:25:07 BST, Paul Randall G3NJV <paulfrandall@...> wrote:


Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


Mike Willis
 

On Thu, Sep 24, 2020 at 09:25 PM, Paul Randall G3NJV wrote:
8481A
There are plenty of HP8484 heads on Ebay for not unreasonable prices between £100 and £200. If you want professional test gear I am afraid you have to pay a little bit of money to get it. Not a lot compared to the new price. My advice is not to but the thermal heads you will blow them up, eventually. The over power capability of the 8484 diode head is much greater relative to it's upper power limit than the 8481.

You can always go for the cheaper solutions, not as accurate, but maybe accurate enough. Usually you don't really need to know the amplifier output exactly. After all you usually then connecting it via coax where the loss is not exactly known, through several connectors and relays to an antenna so you have really no idea what the actual EIRP is to within a dB or so. Knowing the PA power output to better than 2% doesn't really figure against the other errors.

--
Mike G0MJW


Neil Smith G4DBN
 

I have an HP33330B 18 GHz Low-Barrier Schottky detector that I use with a set of Radiall attenuators.  I calibrate it now and then against my 8481A and 8481D heads on an HP436A. I'm thinking of building an interface to it with an A/D converter and PIC and building that into a 435 case with a rechargeable battery and a USB interface for charging, updating the calibration curves and reading the output using a 436A or ACPI emulation.

OK, I could build it into a small case with an e-ink display, but I like the big analogue meter when tweaking things.  I have a 435B that is very clean and works well, so I'd have to find an unloved one to gut and re-purpose.  Having it run on batteries would make it a lot more useful.

Neil G4DBN


On 25/09/2020 11:05, Mike Willis wrote:
On Thu, Sep 24, 2020 at 09:25 PM, Paul Randall G3NJV wrote:
8481A
There are plenty of HP8484 heads on Ebay for not unreasonable prices between £100 and £200. If you want professional test gear I am afraid you have to pay a little bit of money to get it. Not a lot compared to the new price. My advice is not to but the thermal heads you will blow them up, eventually. The over power capability of the 8484 diode head is much greater relative to it's upper power limit than the 8481.

You can always go for the cheaper solutions, not as accurate, but maybe accurate enough. Usually you don't really need to know the amplifier output exactly. After all you usually then connecting it via coax where the loss is not exactly known, through several connectors and relays to an antenna so you have really no idea what the actual EIRP is to within a dB or so. Knowing the PA power output to better than 2% doesn't really figure against the other errors.

--
Mike G0MJW
_._,_._,_



Chris Bartram G4DGU
 

As they were needed for my former work (apologies for the bad language!) I have a number of power meters complete with sensors to 40GHz. While for historical reasons I don't lend instruments, I'd be quite happy to 'calibrate' anyone's home-made sensor.

While I have working a 8485 and a 8482, I also have a damaged 8481 which suffered a gross overload due to my utter stupidity. Although a repair would be possible, I can't now justify the cost of the replacement thermal sensor module.

I have somewhere seen a design for a diode detector module designed as effectively a drop-in replacement for the 8481 electronics (albeit with out the accuracy!). Does anybody have a reference?

73

Chris G4DGU


On 25/09/2020 08:39, geoffrey pike via groups.io wrote:
Hi,
I have used a AD8317 in a non working HP435B and without changing its original state, just use the meter.
From my notes(i actually have them), its performance at 10 GHz (AD8317 spect to 8GHz) shows it
reads about 20 dB low the real value and fairly good to 8GHz. Scaling on the meter is in my case -5dBm FSD and -30dBm mid scale with 55dB range.
The AD8317 does have a -ve output slope, anyway for about a tenner you have a meter that you can abuse and not worry about the thermistor head
cheers
Geoff
GI0GDP

On Thursday, 24 September 2020, 21:25:07 BST, Paul Randall G3NJV <paulfrandall@...> wrote:


Having explored multiple avenues to get a calibrated power reference into my workshop I finally bit the bullet and bought a cheap HP435B non-working power meter for parts for a home-brew power meter. On delivery, I find it in good condition and might even be completely working - just the power switch not illuminating.
So now I'm in the place I dreaded, I'll find it hard to break this classic instrument just for parts so am wondering if anyone out there has a power sensor and cable just lying around unused that they might part with.
An 8481A is the ideal of course but on ebay with cable it is more than my monthly pension so might as well be made of unobtainium. I guess even a long term loan would be good - once I have finished building my projects to venture into the microwave region, the entire instrument could be retasked to a new home.
So, any possibilities out there? I'd be glad to hear.
73s Paul G3NJV


 

Hi Chris,
The article you remember is there somewhere, I looked at it earlier this year, but cannot find it now. I think it was an Italian author, but I could be wrong.
The original circuit is simple enough, but when you read the HP guidance along the lines  "Don't reposition any of the wires, they were put where they are for a reason when we set it up" it is clear that maintaining isolation between the 10V chopper drive and the microvolt detector output is critical to a successful result. I did not find data for the chopper FETs that HP used but surely better are available 40 years on, ie RF types with very low capacitance. I was thinking I might  try an optically driven SPDT FET switch which might also have good isolation.

Ken  G3YKI


Chris Bartram G4DGU
 

Hello Ken,

Thanks for convincing me that I wasn't hallucinating!

I've since been able to find an extensive discussion on the HP/Agilent Equipment list which covered the subject. It provides almost enough information to develop a design, but there are still some unanswered questions. I'll now add the project to the (too) long list of things I want to do! Using an optically driven sampler is a great idea which I'll investigate further as and when.

My first thoughts are that as the sampler is only required to work with DC levels, it should be possible to go down the optical isolation route without too much trouble. The design of the diode detector needed to get sufficient bandwidth/flatness is perhaps a more demanding challenge, but it seems to be simple to separate that from the RF circuitry. I have a couple tunnel diode detectors in my collection, which I believe are spec'ed for 18GHz operation, and it might be possible to use them.

73

Chris G4DGU


Ken G3YKI wrote:

Hi Chris,
The article you remember is there somewhere, I looked at it earlier this year, but cannot find it now. I think it was an Italian author, but I could be wrong.
The original circuit is simple enough, but when you read the HP guidance along the lines  "Don't reposition any of the wires, they were put where they are for a reason when we set it up" it is clear that maintaining isolation between the 10V chopper drive and the microvolt detector output is critical to a successful result. I did not find data for the chopper FETs that HP used but surely better are available 40 years on, ie RF types with very low capacitance. I was thinking I might  try an optically driven SPDT FET switch which might also have good isolation.


Paul Randall G3NJV
 

Chris, Ken
One snippet from that discussion is the part number for the instrument connector, Farnell UK have them (#1321484)

Robin had a pcb started - not sure if this was based on the 8484 schematic?
It must be worth a punt just to make an 8484 amp using modern fets and see how much fiddling with gimmick capacitors it needs - winter is coming!
What about zero drift op-amps with internal commutation? Can they give appropriate noise and input bias requirements?
Have you any part number for the optically coupled sampler in mind Ken?

I don't understand how HP overcame the temperature sensitivity of diode detector in the 8484 sensor, the only temp. sensitive device in the amp is CR1, a schottky diode.
According to the curves given in this document...
(https://www.analog.com/en/technical-articles/integrated-diode-based-rf-detectors.html)
... at higher levels the effect isn't too bad, (convenient because that's where you'd get diode heating from the RF)
However, at 8484 detection levels it looks a mess.
I have an Aertech 18GHz detector so although messy, it might be worth trying to interface this through an 8484 lookalike circuit into the HP436.
Paul



Paul Randall G3NJV
 

Ken, I also read a useful article by an Italian OM this year. Will try and find it again. Paul


Colin Ranson
 

All,

 

I bow to some OM’s knowledge of measurement heads for the HP 432/435 etc , homebrew or otherwise. I will remain on the sidelines looking on with interest as the science is beyond me.   I will get round to firing up my new (old stock ) and unused HP435.

 

Thank you for your posts,

 

Colin de G8LBS.

 

Sent from Mail for Windows 10

 

From: Paul Randall G3NJV
Sent: 27 September 2020 13:26
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] HP Power Sensor wanted

 

Ken, I also read a useful article by an Italian OM this year. Will try and find it again. Paul

 


Robin Szemeti - G1YFG
 

I would have thought modern JFETs with a leakage of well under a nano amp were up to the job.  It is perhaps worth remembering that for "amateur" use, a level of uncertainty in the measurement of perhaps 10% is acceptable. The original HP heads were calibrated and certified to NIST standards and capable of levels of accuracy suitable for certification requirements in demanding situations.  In most cases, an amateur will be satisfied with a lower level of accuracy and a measurement capability to 10 GHz ... maybe less.  What I usually want to know is "does this signal source still produce 5mW after the latest accident" etc, and somewhere between 4.5 and 5.5 mW will be more than sufficient to identify whether it is still functional.

Given those parameters, I think it should be possible to create a diode measurement head, possibly with a built-in attenuator to take it to the 100mW level that will function with an unmodified 432/435/438 etc  allowing for a "proper" head to be attached at some later date.



On Sun, 27 Sep 2020 at 14:26, Colin Ranson <g8lbs@...> wrote:

All,

 

I bow to some OM’s knowledge of measurement heads for the HP 432/435 etc , homebrew or otherwise. I will remain on the sidelines looking on with interest as the science is beyond me.   I will get round to firing up my new (old stock ) and unused HP435.

 

Thank you for your posts,

 

Colin de G8LBS.

 

Sent from Mail for Windows 10

 

From: Paul Randall G3NJV
Sent: 27 September 2020 13:26
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] HP Power Sensor wanted

 

Ken, I also read a useful article by an Italian OM this year. Will try and find it again. Paul

 


--
Robin Szemeti - G1YFG


Andy G4JNT
 

To make a head suitable for use with an unmodified HP432, you'd have to use a similar DC balancing technique.   (Can't speak for teh '435, it uses a different technique)

The 432 uses thermistors and DC unbalance to maintain a constant thermistor temperature and bridge state, so the reduction in DC power is exactly equal to the RF under test.  To make any sort of diode detector / IC replacement head for that circuit is well-nigh impossible

Why not just start from scratch, use your diode / IC into an A/D and PIC + display [other controllers are available].   USe the PIC PWM output to drive an analogue meter if that's what you want

 


On Sun, 27 Sep 2020 at 15:04, Robin Szemeti - G1YFG <robin@...> wrote:
I would have thought modern JFETs with a leakage of well under a nano amp were up to the job.  It is perhaps worth remembering that for "amateur" use, a level of uncertainty in the measurement of perhaps 10% is acceptable. The original HP heads were calibrated and certified to NIST standards and capable of levels of accuracy suitable for certification requirements in demanding situations.  In most cases, an amateur will be satisfied with a lower level of accuracy and a measurement capability to 10 GHz ... maybe less.  What I usually want to know is "does this signal source still produce 5mW after the latest accident" etc, and somewhere between 4.5 and 5.5 mW will be more than sufficient to identify whether it is still functional.

Given those parameters, I think it should be possible to create a diode measurement head, possibly with a built-in attenuator to take it to the 100mW level that will function with an unmodified 432/435/438 etc  allowing for a "proper" head to be attached at some later date.



On Sun, 27 Sep 2020 at 14:26, Colin Ranson <g8lbs@...> wrote:

All,

 

I bow to some OM’s knowledge of measurement heads for the HP 432/435 etc , homebrew or otherwise. I will remain on the sidelines looking on with interest as the science is beyond me.   I will get round to firing up my new (old stock ) and unused HP435.

 

Thank you for your posts,

 

Colin de G8LBS.

 

Sent from Mail for Windows 10

 

From: Paul Randall G3NJV
Sent: 27 September 2020 13:26
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] HP Power Sensor wanted

 

Ken, I also read a useful article by an Italian OM this year. Will try and find it again. Paul

 


--
Robin Szemeti - G1YFG


Paul Randall G3NJV
 

The range of sensors that work with the HP435 (and others) measure the DC out from a thermocouple or a diode, both using a very similar chopper-amplifier to convert to 220Hz before passing to the main instrument.
I have a Boonton power meter with a twin diode, pos & neg output detector which passes the DC direct to the instrument for chopping using a special relay.

The possibilty of a homemade thermocouple head seems extremely remote however some success is being reported using diode detection.

As Robin says, most of the time absolute accuracy is not required, especially since the 435 has that calibrate output. (Although Andy JNT has made the point that 1dB error is the difference between 400 and 500 watts, tricky to not know if assessing PA efficiency.)

Design of a wideband diode head is one thing but reproducing the HP chopper amplifier present in an 8484 head using modern components is maybe not that difficult. The issue seems to be keeping the chopper switching drive (and other leakage and noise) out of an extremely small signal.
The HP sensor heads do not have a power feed - a two phase 180 switch signal is available and HP used proprietary FETs along with tweaking of wire positions to balance out that interference. There is a single BJT in there and the HP manuals give a good description of how it is used.

Andy, you are absolutely right in saying "start from scratch" and I did just that, one project branch used an 18GHz commercial diode detector, another a home made detector and finally a cheap AD8318 log module. All resulted in useful instruments but unfinished due to lack of a power reference - that's why I bought the HP435. BUT, having purchased a cheap "non-working" unit, I found it WAS working apart from a non-illuminating power switch. I am now subject to massive "mission creep" because the prospect of breaking it is not very appealing.

So, a HP compatible detector head for say, +20 to -20 to +/- half dB and couple or 5 GHz would be great for me and probably a widely appreciated bit of work, Owners of precious thermocouple heads could also benefit by preserving them for when really required.

With winter and another lockdown looming, there may be time to experiment.

Cheers Paul
PS probably written up elsewhere but the power switch illuminator is an unobtainium HP replacement module, however it can be dismantled and the 4mm wire end neon bulb inside replaced. Ask how if interested.


Andy G4JNT
 

For quickie wide range microwave measurements I have an HP8473C into a zero input-impedance buffer, measuring the current it delivers into a short circuit.   That is linear (in power / voltage terms,   so therefore square law) over the range from -50dBm, below which it is near-impossible to balance the opamp for zero offset, up to around -5dBm where compression creeps in.   The opamp has switchable feedback resistors in 5dB (3.16 ratio) steps so an analogue meter can be kept in its useful range.

I calibrated it against my HP432 and also a synthesized signal generator with a decent output level attenuator and output level setting when first built, well over 20 years ago.  Checked from time-to-time and the readout has never been out by more than half a dB.     Biggest accuracy problem is nulling out DC offsets, which come not just from  the op-amp but also from bimetallic effects in the cable to the diode head, along with any DC drop in ground loops.   To avoid the latter I  run the power meter off internal batteries.   A DC -DC converter would do as well, but can't be bothered.   

Years after building that meter, I got another two HP8473C detectors, and tried them on the meter.  It was shocking just how far the calibration was then out.   What is ostensibly the same diode, needed in one case a 60% change in gain and in the other more than a factor of two.   So the original detector is now firmly fixed with self-amalg tape round its BNC lead to the electronics.

I subsequently made up an identical unit using one of the new detectors, but this time with an A/D converter and PIC to USB/COM Port interface for PC logging.   However, I 'forgot' about the ground loop problem, so although it works OK on laptops with no DC path to earth via the  PSU,  on the shack desktop, forget it.   The correct technique would have been to use an optically isolated serial interface, and build in a DC-DC converter (I was using the USB 5V for power) .   A careless bit of design work there!

SO all that gives quite accurate power measurement down to -50dBm, good enough for antenna range tests over a few hundred metres with dish type antennas.  For high powers there's the HP432 with a 20dB 5 Watt 12GHz attenuator glued to the detector head, having blown up two of them over the years.   

For more sensitive power measurement , (down to noise level) I built a test receiver , an image-cancelling downconverter with a tuneable Fract-N synth LO covering 70 - 300MHz, but mainly for transverter IF at 144 IF.   That converts to 21.4MHz where a standard FM filter gives a 10kHz working bandwidth.  After that, into a switchable gain amplifier chip then an AD8307 log-power device.   Taking the AD8307 output voltage measured in an A/D + PIC, and control of the precision gain chip gives seamless power readout on an LCD and bargraph from -120 up to +10dBm.   I can connect to a transverter (whose RF to IF gain has been measured) , take it up a hilltop and see the real  actual RF level coming out of my antenna.   More on that measurement Rx in the November 2016 RadCom, (page 171 of a certain book RSGB have just published :-)



On Sun, 27 Sep 2020 at 16:52, Paul Randall G3NJV <paulfrandall@...> wrote:
The range of sensors that work with the HP435 (and others) measure the DC out from a thermocouple or a diode, both using a very similar chopper-amplifier to convert to 220Hz before passing to the main instrument.
I have a Boonton power meter with a twin diode, pos & neg output detector which passes the DC direct to the instrument for chopping using a special relay.

The possibilty of a homemade thermocouple head seems extremely remote however some success is being reported using diode detection.

As Robin says, most of the time absolute accuracy is not required, especially since the 435 has that calibrate output. (Although Andy JNT has made the point that 1dB error is the difference between 400 and 500 watts, tricky to not know if assessing PA efficiency.)

Design of a wideband diode head is one thing but reproducing the HP chopper amplifier present in an 8484 head using modern components is maybe not that difficult. The issue seems to be keeping the chopper switching drive (and other leakage and noise) out of an extremely small signal.
The HP sensor heads do not have a power feed - a two phase 180 switch signal is available and HP used proprietary FETs along with tweaking of wire positions to balance out that interference. There is a single BJT in there and the HP manuals give a good description of how it is used.

Andy, you are absolutely right in saying "start from scratch" and I did just that, one project branch used an 18GHz commercial diode detector, another a home made detector and finally a cheap AD8318 log module. All resulted in useful instruments but unfinished due to lack of a power reference - that's why I bought the HP435. BUT, having purchased a cheap "non-working" unit, I found it WAS working apart from a non-illuminating power switch. I am now subject to massive "mission creep" because the prospect of breaking it is not very appealing.

So, a HP compatible detector head for say, +20 to -20 to +/- half dB and couple or 5 GHz would be great for me and probably a widely appreciated bit of work, Owners of precious thermocouple heads could also benefit by preserving them for when really required.

With winter and another lockdown looming, there may be time to experiment.

Cheers Paul
PS probably written up elsewhere but the power switch illuminator is an unobtainium HP replacement module, however it can be dismantled and the 4mm wire end neon bulb inside replaced. Ask how if interested.


Paul Randall G3NJV
 

Andy
Many thanks for taking the time to share all that, I'll digest it later this evening once I've lit the fire and poured a glass of wine.
I do remember reading about your measurement receiver, something I've always intended to build.
I wonder if my Aertech D18Z detector is a low barrier type, the data sheet doesn't use that term.
One thing new to me - operating the detector into zero impedance and measuring current, exactly the opposite of what I have been doing. I'll look again at the HP 8484 schematic to see what it does. Is there an easily available reference on that technique so I can swot up on it?

Best regards, Paul


Andy G4JNT
 

No, there isn't all that much about it.  G3YGF, many decades ago did a lot of work on it (Julian, are you here ?)   and back then he had a proper zero-bias diode which IIRC offered something like 70dB of range.   I've seen the ORIGINAL meter he built with it, still in use today.   And the age of that makes my 25 or so year old version a mere newcomer.   Being zero-bias, it had a much lower dynamic resistance making zero offset on the opamp easier.    The HP detector diode gives lower dynamic range, but still enough to work with - especially if you look around and get yourself a set of decent attenuators.




On Sun, 27 Sep 2020 at 18:55, Paul Randall G3NJV <paulfrandall@...> wrote:
Andy
Many thanks for taking the time to share all that, I'll digest it later this evening once I've lit the fire and poured a glass of wine.
I do remember reading about your measurement receiver, something I've always intended to build.
I wonder if my Aertech D18Z detector is a low barrier type, the data sheet doesn't use that term.
One thing new to me - operating the detector into zero impedance and measuring current, exactly the opposite of what I have been doing. I'll look again at the HP 8484 schematic to see what it does. Is there an easily available reference on that technique so I can swot up on it?

Best regards, Paul