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On Tue, Sep 29, 2020, 21:25 Arv Evans <arvid.evans@...> wrote:
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Rene
 

??


Hans Summers
 


Rubens Kamimura
 

Hans Summers, 

I received and assembled the PA 50W kit for CQX (CW), I would like to comment about the possibility of using the PA-50W qrp-labs, in FT8, JS8Call and RTTY modes on the uBITX v4 radio output.
73's de Rubens py2pvb.


Em qua., 30 de set. de 2020 às 01:55, Hans Summers <hans.summers@...> escreveu:


Curt
 

Rubens

The bias on this PA is only suitable for cw. Some significant mods are required for linear operation needed for these other modes. The amplifier must be modified for steady current flow. I don't know that anyone has reported those modifications.

73 curt


Evan Hand
 

Rubins,
The amp is designed for Class C operation.  That does not support any of the modes that you list.

I will be interested to see Hans' reply, however, Hans has stated in the past that it could be biased for linear operation, just that he has not done it, nor would he provide support for the effort.  The basic design is similar to other push-pull designs available from multiple sources.  Those designs have single biasing networks for the MOSFETS, as does the PA 50W.  This would indicate that it is possible.

There are also messages on the QRP-Labs Groups.io lists that suggest others have increased the bias and gotten good signal reports.  The messages did not report the results of any signal purity tests.
https://groups.io/g/QRPLabs/message/52355

To sum it up, it can be done.  I would not recommend it to anyone who is not familiar with testing to verify compliance and that has access to the equipment to do that testing.  At less than 15 watts, the purity of the signal is not as noticeable as at 50 watts.  The other point is to make sure your v4 uBITX is in compliance before amplifying the output.  The harmonics and spurs being feed into the amp will be amplified as well.  The uBITX v4 has both of those issues as delivered by HFSignals.

Recently a lower cost spectrum analyzer, the TinySA, has been made available.  This device at less than $65 delivered could be an instrument to verify operation, with the proper dummy load, tap, attenuator, and the knowledge on how to use them.
https://www.tinysa.org/wiki/

The above are my thoughts and may contain errors.  Please verify before you make choices based on them.
73
Evan
AC9TU


Rubens Kamimura
 

Evan, AC9TU,
 
At first I was upset, but I understand that this low cost amplifier was designed for operation only for CW. In the group there are modifications of the PA of uBITX v4 that will meet my expectations, I will experiment with parts of this PA 50W (qrp-labs).

I accept suggestions for modifications of this PA 50W, for operating digital modes (FT8) in 40m (7074).

73's
Rubens 
py2pvb


Hans Summers
 

Hello Rubens

Yes, the 50W amp kit http://qrp-labs.com/50wpa was designed for CW - specifically the QCX series of rigs though it would work with other QRP transceivers too.

However, as others have said... the circuit design is similar to many other push-pull amps around, which are designed as linear amps. The difference is largely in the bias adjustment. 

For CW it works best in Class C for which you dial in a zero idle current bias setting. For Class AB increase the bias voltage until with no signal, current flows through the MOSFETs. I'd suggest aiming for a value of 250mA as a starting point. 

As others said, it is definitely a good plan to make measurements on the output to check the linearity. IMD3 AND IMD5 would be nice to know. 

Always follow any amp (even a professional linear) with a low pass filter, because no amps are perfect. 

I do plan to do experiments on the 50W Amp kit as a linear but I haven't had an spare moment for it yet. 

73 Hans G0UPL 


 

Hi Rubens,

In order for the amp to operate correctly in linear mode, you would need to change the bias circuit.  Looking at the schematic, the bias is adjusted simulaneously for both transistors; this would be OK for Class C.  However for a linear class, the transistors would need to be balance as best as possible.  This cannot be done with a single bias circuit because MOSFETs have differing turn-on thresholds.  It would not be too difficult to change, but would take some cutting of tracks and adding of the needed parts.  Simply look at the bias circuit for the WA2EBY amp and copy that.

73,  Mark


Evan Hand
 

Mark,
In a literal sense, you are correct.  However, I have seen where the selection of parts from the same batch and careful adjustment of the bias to hit the midpoint of the two devices to work.  It is also the same bias circuitry as other successful linear amps.  The need to verify linear operation is paramount to ensure compliance.  This cannot be assumed without verification.  Having separate bias controls would make it easier to ensure compliance, but not a true requirement.

Without testing, we do not know who is correct.  Validation of compliance is required regardless of the bias method.

These are my opinions and could be in error.  Each person needs to verify for themselves with actual data.
73
Evan
AC9TU


 

Hi,

I was speaking of modifying Ruben's current amp (without ordering and trying to match transistors).  Anyway, the far majority of MOSFET push-pull linear amp designs I've seen use a bias circuit for each transistor which makes sense to me.  What successful linear MOSFET amps with separate final transistors have used a single bias supply?

73


Ashhar Farhan
 

Fabulous write up on the 50 watt amplifier from Hans. It is a master class on so many different issues of linear amplifier design.
Next week, I will be getting on to a similiar work. 
Hans, I was curious about the LPF performance. Why is there a notch beyond the cutoff and then the leakage at higher frequencies? This is usually traced to stray inductance on the ground side. Stitching up the ground with more PTH should help. 
What make and kind of capacitors do you recommend for the LPFs?
Commendable work, absolutely brilliant.

73, f

On Thu 1 Oct, 2020, 6:58 AM Mark - N7EKU, <n7eku@...> wrote:
Hi,

I was speaking of modifying Ruben's current amp (without ordering and trying to match transistors).  Anyway, the far majority of MOSFET push-pull linear amp designs I've seen use a bias circuit for each transistor which makes sense to me.  What successful linear MOSFET amps with separate final transistors have used a single bias supply?

73


Bill Cromwell
 

Hi,

Validation of compliance is much easier with the recent introduction of the tinySA. That device can bought new (there aren't any truly old ones) under $100 plus shipping. Depending where you buy it the price is 50 or dollars. It will cost the average more in time learning how to use it than acquiring it.

At one time, our RF transmit power was specified as the DC input power to the final amplifier plate circuit. Most amateurs did not have any means to measure the output power. When it became practical to measure the output power the specification was changed to use that capability.

I think it is now the same checking spectral purity. The instrument is just too easily acquired. Amateurs who really cannot afford ~100 dollars for a private, personal tinySA can cooperate with others to share one. There will be some other bits needed to use one - mainly a set of appropriate power attenuators. With some care, those could be home brewed. You cannot just plug your transmitter output straight into any spectrum analyzer - not even a QRP transmitter. The SA will be destroyed.

Start here:

https://groups.io/g/tinysa

That will lead you to the how-to documents including how to buy one. Look in the WIKI on that group.

73,

Bill KU8H

bark less - wag more

On 9/30/20 6:58 PM, Evan Hand wrote:
Mark,
In a literal sense, you are correct.  However, I have seen where the selection of parts from the same batch and careful adjustment of the bias to hit the midpoint of the two devices to work.  It is also the same bias circuitry as other successful linear amps.  The need to verify linear operation is paramount to ensure compliance.  This cannot be assumed without verification.  Having separate bias controls would make it easier to ensure compliance, but not a true requirement.
Without testing, we do not know who is correct.  Validation of compliance is required regardless of the bias method.
These are my opinions and could be in error.  Each person needs to verify for themselves with actual data.
73
Evan
AC9TU


Evan Hand
 

Mark,
Here is one:
https://www.qsoshack.com/make-a-45-watt-linear-amplifier/

There are many others on the internet.  The easiest way is to use a pair of MOSFETS in a common package with a heat spreader so that they are by design from the same batch.  These are of course more expensive and can cost more than the other parts of the amplifier combined.

Again I will state that it is possible, not that it always works.  The main point was that IF you are experimenting with devices outside of their design use, THEN you need to verify proper operation.  That is also why I gave the link to the TinySA (see Bill KU8H message in the thread).  This device makes the modification process possible.

Bill,
I totally agree with your comments.  In addition to the TinySA, I have found this tap to be low cost and easy to build:
http://www.ad5x.com/images/Articles/Tap50dbRevA.pdf

I did find that I had to adjust the capacitor slightly, as my NanoVNA (Another inexpensive tool well worth the money) showed a rise in the measured output above 30 MHz.  This made some of the harmonic measurements less accurate, however in the correct direction for purity assurance (It would report a higher than actual signal level).

As always, these are my opinions.  Others can do with them as they see fit, and they are offered as best knowledge on my part at this time.  There may be better information available from others.
73
Evan
AC9TU


Ashhar Farhan
 

In addition to the tinysa, the antunio does a fairly decent job. I have almost stopped using my rigol after the v2.2.upgrade.
TinySA has a noisy floor. That hampers precise iip3 measurements.
- f

On Thu 1 Oct, 2020, 3:54 PM Evan Hand, <elhandjr@...> wrote:
Mark,
Here is one:
https://www.qsoshack.com/make-a-45-watt-linear-amplifier/

There are many others on the internet.  The easiest way is to use a pair of MOSFETS in a common package with a heat spreader so that they are by design from the same batch.  These are of course more expensive and can cost more than the other parts of the amplifier combined.

Again I will state that it is possible, not that it always works.  The main point was that IF you are experimenting with devices outside of their design use, THEN you need to verify proper operation.  That is also why I gave the link to the TinySA (see Bill KU8H message in the thread).  This device makes the modification process possible.

Bill,
I totally agree with your comments.  In addition to the TinySA, I have found this tap to be low cost and easy to build:
http://www.ad5x.com/images/Articles/Tap50dbRevA.pdf

I did find that I had to adjust the capacitor slightly, as my NanoVNA (Another inexpensive tool well worth the money) showed a rise in the measured output above 30 MHz.  This made some of the harmonic measurements less accurate, however in the correct direction for purity assurance (It would report a higher than actual signal level).

As always, these are my opinions.  Others can do with them as they see fit, and they are offered as best knowledge on my part at this time.  There may be better information available from others.
73
Evan
AC9TU


Bill Cromwell
 

Hi Evan,

Thank you for that PDF re the tap. I know I saw that somewhere before but I had not captured it on my hard drive. I have it now.

73,

Bill KU8H

bark less - wag more

On 10/1/20 6:24 AM, Evan Hand wrote:
------snip---
Bill,
I totally agree with your comments.  In addition to the TinySA, I have found this tap to be low cost and easy to build:
http://www.ad5x.com/images/Articles/Tap50dbRevA.pdf
I did find that I had to adjust the capacitor slightly, as my NanoVNA (Another inexpensive tool well worth the money) showed a rise in the measured output above 30 MHz.  This made some of the harmonic measurements less accurate, however in the correct direction for purity assurance (It would report a higher than actual signal level).
---snip----
73
Evan
AC9TU


Rubens Kamimura
 

Mark, and group.

Very grateful for the excellent tips for modifying the 50W PA (Hans - qrp-labs), let's work on this.

fraternally,
73's
Rubens
py2pvb


Bob Benedict, KD8CGH
 

Another option for a proven, robust, all mode amp is the Hardrock 50 kit. This is a semi kit with SMD devices installed, similar to the uBITX. There is also a companion auto tuner available that fits in the enclosure.

https://www.eham.net/reviews/view-product?id=10839


Hans Summers
 


Hi Farhan

> Hans, I was curious about the LPF performance. Why is there a notch beyond 
> the cutoff and then the leakage at higher frequencies? This is usually traced to 
> stray inductance on the ground side. Stitching up the ground with more PTH 
> should help. 

Have a look at the PCB OM... there is extensive groundplane both sides and it is heavily stitched together with plated through holes at very tight intervals. I spend many weeks on my PCB layouts. 

Once you get up into VHF and with a high performance spectrum analyzer with 100dB dynamic range you find that the whole measurement is sensitive to just about everything. It's more likely to do with the cables and how I connected the spectrum analyzer with tracking generator to the board, than to do with the actual reality on the board. 

> What make and kind of capacitors do you recommend for the LPFs?

You have to use NP0 capacitors if using ceramics, not for the low temperature coefficient, but for the Class-I dielectric! They need to have sufficient voltage rating for the job. And buy from a reputable source so as to be sure they really are NP0. I get mine shipped from Digikey US. I have 23,900 en route right now, in various values! 

73 Hans G0UPL
http://qrp-labs.com


Ashhar Farhan
 

Hans,
Got it!

On Thu 1 Oct, 2020, 8:02 PM Hans Summers, <hans.summers@...> wrote:

Hi Farhan

> Hans, I was curious about the LPF performance. Why is there a notch beyond 
> the cutoff and then the leakage at higher frequencies? This is usually traced to 
> stray inductance on the ground side. Stitching up the ground with more PTH 
> should help. 

Have a look at the PCB OM... there is extensive groundplane both sides and it is heavily stitched together with plated through holes at very tight intervals. I spend many weeks on my PCB layouts. 

Once you get up into VHF and with a high performance spectrum analyzer with 100dB dynamic range you find that the whole measurement is sensitive to just about everything. It's more likely to do with the cables and how I connected the spectrum analyzer with tracking generator to the board, than to do with the actual reality on the board. 

> What make and kind of capacitors do you recommend for the LPFs?

You have to use NP0 capacitors if using ceramics, not for the low temperature coefficient, but for the Class-I dielectric! They need to have sufficient voltage rating for the job. And buy from a reputable source so as to be sure they really are NP0. I get mine shipped from Digikey US. I have 23,900 en route right now, in various values! 

73 Hans G0UPL
http://qrp-labs.com