New QCX Mini 17M completed - expected rx sensitivity (minimum detectable signal level)?


David Feldman
 

I've just completed assembly and initial testing (using the procedures in manual 1.05) of 17M QCX Mini.

I could not find a specification for expected sensitivity or MDS on 17M band; I connected HP 8660 synthesizer and find minimum detectable signal about 1 uV which seems rather insensitive. I've not tested transmit yet. Supply power is 10.5V from batteries.

Is there any spec or expected value so that I can determine whether I should troubleshoot?

Thanks,

Dave


Jim Allyn - N7JA
 

According to measurements by WA4MZS found at the bottom of this page:    http://qrp-labs.com/qcxp.html    sensitivity is -121 dBm.  According to the ARRL review found here:    http://qrp-labs.com/images/qcx/ARRLAug2019.pdf    sensitivity is -128 dBm.  Those figures are both on 40 meters.

I would expect the sensitivity to be closer to the -128 dBm figure than the -121 dBm figure.


Jim Allyn - N7JA
 

-121 dBM = .2 uV.    -128 dBm = .09 uV.


Shirley Dulcey KE1L
 

Performance of any given build is going to depend a bit on things like how well optimized the low pass and band pass filters are. Extra losses there will degrade the MDS number a bit. And components are not all identical.

The MDS of the QCX is determined by five things in the circuit: The low pass filter, the T/R switch, the band pass filter, the QSD, and the first stage of op-amps (IC5A and IC5B). Anything after that first gain stage will have only a very small effect, which is why the QCX Mini can use lower performance op amps in the later stages without significant loss in performance while enjoying the drop in power consumption from the change in op amps.

All else being equal, I would expect performance on 17m to be a bit worse than on 40m because losses in the QSD will increase. The loss in the T/R switch Q5 might also edge up a tad. The band noise level is also lower on 17m. On the other hand, you're probably not using an antenna with substantial losses on 17m. So long as you can hear the band's noise floor if you turn up the volume setting, your rig is sensitive enough.


On Thu, Jul 22, 2021 at 8:40 PM Jim Allyn - N7JA <jim@...> wrote:
According to measurements by WA4MZS found at the bottom of this page:    http://qrp-labs.com/qcxp.html    sensitivity is -121 dBm.  According to the ARRL review found here:    http://qrp-labs.com/images/qcx/ARRLAug2019.pdf    sensitivity is -128 dBm.  Those figures are both on 40 meters.

I would expect the sensitivity to be closer to the -128 dBm figure than the -121 dBm figure.


David Feldman
 

Thanks Jim and Shirley, that is all helpful.

I removed the radio from it's case, connected the HP 8660 back up, and found that when AF gain is wide open that peaking the trimmer capacitor isn't as straightforward as I had originally expected - there's a weak background audio tone (that seems to match the peak in the rx audio filter) that I was peaking on, rather than the signal from the 8660. With some more care in trimmer adjustment I have MDS around 0.3 uV (-124 dBm?), so that seems plausible. More testing tomorrow.

Thanks again,

Dave


ajparent1/KB1GMX
 

For many decades the acid test for RX senstivity was:

Audio output with 50 ohm load.
With not other changes audio output when connected
to a dipole or other known working antenna.

Experience is if the second case has a noticeable increase
in noise to the ear the sensitivity has proven more than
adequate for the band.

Allison
-------------------------------
Please reply on list so we can share.
No private email, it goes to a bit bucket due to address harvesting


David Feldman
 

Thanks ---- I've been able verify 17M noise floor elevates when antenna attached, so at this point I'm going to declare victory on this issue for time being. TX power seems to be consistent with expectation. Brief foray onto 20 meters was successful as well.

One other question/concern - the radio is very (very) sensitive to electrical noise arriving on the DC power jack; has anyone discussed replacing (or supplementing, if enough space in which to work) the input diode with an inductor and a couple of shunt capacitors to reduce noise pick-up arriving via DC power source?

Thanks for any thoughts on this,

Dave


Jim Sheldon
 

Dave,
I'd be very careful replacing that diode with the inductor - the diode is there to prevent accidental polarity reverse power input from destroying the MINI. I'd seriously consider getting a power supply that was either an analog supply or guaranteed RF noise free from the manufacturer. Much safer and well worth the investment in the long run.

Jim, w0EB

------ Original Message ------
From: "David Feldman via groups.io" <wb0gaz=yahoo.com@groups.io>
To: QRPLabs@groups.io
Sent: 7/23/2021 9:54:27 AM
Subject: Re: [QRPLabs] New QCX Mini 17M completed - expected rx sensitivity (minimum detectable signal level)?

Thanks ---- I've been able verify 17M noise floor elevates when antenna attached, so at this point I'm going to declare victory on this issue for time being. TX power seems to be consistent with expectation. Brief foray onto 20 meters was successful as well.

One other question/concern - the radio is very (very) sensitive to electrical noise arriving on the DC power jack; has anyone discussed replacing (or supplementing, if enough space in which to work) the input diode with an inductor and a couple of shunt capacitors to reduce noise pick-up arriving via DC power source?

Thanks for any thoughts on this,

Dave


tony.volpe.1951@...
 

Just to second Jim's remark; when I first fired up my original Classic QCX, I used a wall wart I had lying around. Everything worked like a charm, but people were calling me and I could barely read them. My first few qsos I was getting 559 reports from people 500 miles away on forty and I was struggling to hear them under the hash.I tried other switching supplies, from old computers, with more or less similar results and soon bought a traditional type, small, linear power supply of suitable output power for about £45. What a difference. It was like a different radio, I could easily read the faintest signals.

G0BZB 
Tony


David Wilcox K8WPE
 

With QRP I don’t even think of AC power supplies. 12 - 14 v batteries are so easily available and last so long that is all I use. Recharge occasionally. I get free 12 v 7 amp gel cells from the hospital when they change them out on a regular basis. They have to pay the recycler to take them so are very happy to give them away to hams and other interested parties.  Someday I will hook them up to my solar panel and have automatic charging. Maybe the 5th Tuesday of next month.  

Most wall warts are cheap and a pain and the lap top ones that are good are too high a voltage.

Dave K8WPE

David J. Wilcox’s iPad

On Jul 25, 2021, at 4:06 AM, tony.volpe.1951@... wrote:

Just to second Jim's remark; when I first fired up my original Classic QCX, I used a wall wart I had lying around. Everything worked like a charm, but people were calling me and I could barely read them. My first few qsos I was getting 559 reports from people 500 miles away on forty and I was struggling to hear them under the hash.I tried other switching supplies, from old computers, with more or less similar results and soon bought a traditional type, small, linear power supply of suitable output power for about £45. What a difference. It was like a different radio, I could easily read the faintest signals.

G0BZB 
Tony


Rob Giuliano
 

I agree with David (K8WPE).
With a little work, you can usually find free, used 12V backup batteries from businesses that are required to change them on a time basis.
Hospitals are probably the most common, but some business which require emergency lighting also use that practice.
My favorites were the 24 volt ones which were typically 2x 12v batteries in series with a fuse between them.
I am not ure why the fuse was between them, but when reqired in parallel, I had a 14 Ah battery or 2x 7 Ah, if you could get them apart.

A friend was close with a manager at a "Battery Plus" store.  He would bring home various 12v betteries for us, and take them back if they didn't work out.