Keyboard Shortcuts
Likes
- QRPLabs
- Messages
Search
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Good. Frankly, there's not much else to look for practical purposes. We already have enough clarity in this area. |
Re: AGC for the QMX?
I'm ok with keeping one hand on the volume but I did think about making a simple passive AGC circuit. If I use a LND150 (depletion + enhancement mode MOSFET, normally halfway on when VGS=0V) and a Schottky diode to make a passive AF shunt AGC, I just need a simple audio amp to complete the whole thing. It's similar to QCX AGC module but the gate bias circuit becomes unnecessary. |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Nope...
toggle quoted message
Show quoted text
It's not difficult to just make multiple simulation runs with different input frequencies to get a good idea of the response at baseband. On Sun, Oct 22, 2023 at 9:13?AM Ryuji Suzuki AB1WX <ab1wx@...> wrote:
|
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Are you guys now trying to feed white noise to the mixer and do the spectral analysis of the I/Q raw signal output for the mixer's frequency response??? (I don't recommend fake impulse for this.) |
Re: AGC for the QMX?
¿ªÔÆÌåÓýGood advice! I had the blast in my phones as you did. I too have had tinnitus for 50 years. Too much time in front of an elementary band!¡®73 Jim WX8J On Oct 22, 2023, at 8:56 AM, K9NUD-Steve <k9nud@...> wrote:
|
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Evan, hmmm...
toggle quoted message
Show quoted text
I read the same thing and come to the opposite conclusion! I take it to mean that truly raw IQ data is conveyed, real baseband data centered about zero, and not data that has been subject to any internal SDR routines, including conversion to a 12KhHz IF. JZ On Sun, Oct 22, 2023 at 8:39?AM Evan Hand <elhandjr@...> wrote:
|
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
John,
Here is a section from the latest QDX operating manual: IQ Mode
When IQ mode is enabled, the raw I and Q channels from the ADC are fed to the USB soundcard
directly, without any demodulation. This is suitable for people wishing to experiment with using
QDX as an SDR front end, with PC SDR software to demodulate I and Q channels.
IQ Mode is not suitable for use with WSJT-X and other Digi mode programs. I would interpret this to mean a 12kHz baseband signal is being sent to the PC through the USB soundcard emulation. 73 Evan AC9TU |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
I should say not just the DC but probably 1/f noise, too. The 1/f noise is probably one of the leading enemies of analog/RF engineers. |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
There probably is a DC component there because of the differential amplifier and the LPF. Any slight imbalance of the input bias, offset, gain, etc. could create a small DC. It's tiny but we are also looking at receivers capable of weaker than -130dBm. |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Alan, just a guess here as I have nil experience with QDXs IQ mode. ?My guess though is that IQ samples that have not benefitted from the superhet conversion to 12 KHz IF that? the QDX internal SDR performs are being sent to your SDR software and that SDR software is responsible for nulling the DC spike. The superhet conversion would moot the issue. This would be a great place for Hans or someone else with deeper knowledge to chime in. JZ On Sun, Oct 22, 2023, 7:37 AM Alan G4ZFQ <alan4alan@...> wrote: On 22/10/2023 09:43, John Z wrote: |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Mine is NanoVNA V2 Plus4 but high impedance measurement is reasonable. When I measured the velocity factors of my coax cables, I measured with the other end open, shorted, and mismatch terminated and calculated the velocity factor from the frequencies that created impedance extremum. They agreed fairly precisely. Since a lot of hams are using NanoVNA mostly as an antenna analyzer, so even a cheap counterfeit might do, but there are situations where the latest model helps. Based on everything we've seen here, QMX's BPF architecture is very reasonable. Even a very low Q series resonance BPF is hugely beneficial because the mixer's input impedance has a large synergistic effect to sharpen the overall preselector performance. If anyone still argues that the BPF is unnecessary, taking advantage of the mixer's intrinsic band-pass filter characteristics requires that the source impedance be high enough all across the stopband frequencies. A series-resonance BPF, even if the Q is very low, will do exactly that. That does not mean that the stray capacitance issue and other existing issues are gone. But solutions to those things should best be done without compromising the preselector performance. |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
On 22/10/2023 09:43, John Z wrote:
from what you described I think you are seeing the annoying DC offset at the center frequency that is common to SDRs and quadrature sampling detectors.John, Yes, sorry some devices do have a big DC spike. I'd have expected the QDX spike to be small but I guess it is not. 73 Alan G4ZFQ |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
A couple points about the NanoVNA. The test signal it presents is nominally about 0 dbm, so it is really quite strong. ?Ryuji is correct when he says that LO leakage would need to be quite large to interfere with that. Inside the? NanoVNA, the 50 ohm bridge circuit and the limited dynamic range of processing of the return signal pose some limits on what level of impedance can be measured with accuracy. Measurements above about 300 ohms become questionable. Still, it's a hugely useful device! Finally, as mentioned before, the Tayloe circuit is incredibly robust and can function very well even if big mismatches occur at its input. We should save our worry for elsewhere. JZ KJ4A? On Sun, Oct 22, 2023, 6:57 AM Ryuji Suzuki AB1WX <ab1wx@...> wrote:
|
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Agreed, attaching TinySA Ultra or something to the mixer input port should tell if there is any significant LO leak to the input port. But I doubt it is large enough to affect the VNA, as the clock is only driving the counter and decoder. It's not a typical SBM or DBM situation. Also, if there is a strong feed through, the VNA measurements would be affected by more frequencies not just one point. But it is still a possibility. More likely, all it takes to see a very high impedance at the input port is precise lining up of the LO and VNA's frequencies. However, we are getting sidetracked to an unimportant matter here. That peak is irrelevant in actual receiver where the mixer is driven by a source of relatively low impedance. Also, in QMX, we care about the IF frequency offset and its image frequency not the LO frequency like DC receivers. Also, keep in mind we are only looking at the input impedance and not the overall mixer response. The traces we saw are not the overall frequency response of the mixer. If the signal source is ideal current source with infinite impedance, those two responses may be proportionate within some limits. We are driving the mixer with a source with finite (in fact, low) and varying impedance. The actual frequency response is expected broader near the receiving frequency but narrower farther away than what appears on the impedance traces, thanks to the BPF. |
Re: Repairing QCX+ PCB
The problem of not clearing the via can be troublesome, I have found sometimes, as strange as it may sound, melt a small dab of solder back on the connection, and go at it again with a clean piece of wick, patience pays off.? Working pcb repair for many years, I've seen my share of ruined boards with cold solder suckers.? Back in the tube and large component pcb days, solder suckers were king. But today, with fragile thread size lands wicking and as an alternative,? ?the rework station, with vacuum extractors seem to be a little better, and are popular as they are heated.??
Regards wb7ond |
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
Alan, from what you described I think you are seeing the annoying DC
toggle quoted message
Show quoted text
offset at the center frequency that is common to SDRs and quadrature sampling detectors. and your software appears to be trying to null it out. If you have an oscilloscope or a spectrum analyzer it might be interesting to attach it to the antenna terminals of QDX, with transmit disabled of course, and see if you can observe or measure the LO feedthrough. If it is present, it will work its way backward through the detector balun, the BPF and muxes, the T/R switch, and the LPF and be clearly observable at the BNC connector. JZ On Sun, Oct 22, 2023 at 5:25?AM Alan G4ZFQ <alan4alan@...> wrote:
|
Re: QDXH Tayloe Zin sweep 14074 Mhz
#experimental
#qdx
#qmx
On 22/10/2023 07:58, John Z wrote:
The double balanced Tayloe circuit does a great job of suppressing LO feedthrough.John, I've just tried IQ mode on a QDX, I think much the same circuitry? I do not seem to have any old soundcard SDR program with no automatic cancelling, or one that it can be turned off. But I do see a swiftly decreasing spike of maybe 40dB in the centre when I shift the LO. 73 Alan G4ZFQ |
Re: Repairing QCX+ PCB
A few thoughts
|