Sideband Selection – QSD Receiver

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I had about 15 minutes to spend on the project today, so I got the sideband selection function working. Since it’s just a matter of swapping the I and Q LO phases, and those phases are defined in the Si5351 software, it was merely a matter of a few lines of code. With a signal tuned in, touching the Mode button on the touch display makes the signal go away and appear again when you tune to the other sideband. It’s an interesting way to qualitatively assess the opposite sideband suppression.

If you come from a crystal filter radio world, you’re probably used to the character of the noise changing when you switch from USB to LSB. This is because the crystal filter bandwidth is not symmetrical and you hear more noise when the BFO is tuned on the lower  skirt than on the upper skirt, or vice versa. With the phasing mode of demodulation, that doesn’t happen and the noise is absolutely the same no matter the mode. Neat!

Now I have to decide which sideband is appropriate for receiving CW signals.

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All Systems are Go – QSD Radio

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Over the weekend I installed the 8-pole audio lowpass filter as well as a temporary 40m band-pass filter at the front-end. Already I can hear the difference in bandwidth and absence of QRM from off-channel signals. I haven’t rigorously tested it yet, But I’ll do that this weekend.

I listened to a contest on Saturday using a little 3″ speaker laying on the bench. Sensitivity seems comparable to my TS-450 and audio quality is good, given the compromise speaker. Sunday, the band was dead until evening.

I wrote a little dissertation on my experience with this rendition of Tayloe detector. You can find it here:

More tests and specs to follow.

AM Broadcast Bleed thru – QSD Receiver

I reported in a previous post that a nearby 5000W AM station was bleeding through in the microvolt level. Today, I threw together a quick and dirty 40M bandpass filter. That solved the BCI problem, but in my haste, I somehow blew up the Nano board for the 3rd time. The receiver’s working now but I’ll have to put in an order for 5 or 6 more Nanos.

Quick and dirty 40M bandpass filter

Ironically, now that the BCI is gone, the band is somewhat dead. It does pick up signals though. Yesterday there was a contest and I heard lots of CW and SSB signals. It’s pretty broad though. Gotta get that 8-pole audio lowpass filter installed.

Switching Gears – QSD Receiver Software

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I found another TFT touchscreen in my ‘archive’ of parts, so I took the opportunity to switch gears for a while and work on the firmware system. I’ll probably do that for a couple more days, then back to hardware.

The tuning knob in this system is implemented with a rotary encoder that has a push-button switch in the shaft. The tuning resolution can be set anywhere from single Hz per encoder step to single Mhz per encoder step. Pushing the knob in while turning it moves an underline cursor to show which digit is being controlled. Pushing the button and holding it without turning for about 2 seconds saves the whole radio state to EEPROM so the radio always powers up in the state you left it in. Pushing the knob and holding it for about 5 seconds clears all the digits lower than the cursor position to zeros. This is all working now. Another encoder will be added to control the volume of an audio power amp with voltage-controlled-gain via a DAC. This will be the underpinning of my intended software defined feed-forward audio AGC.

Touching any of the on-screen soft-buttons toggles to the next control state state of that button. Touching the orange “Menu” button redefines all the buttons with a different set of control functions.

The S-Meter is a moving bar that will be calibrated in accurate S units.

All the firmware infrastructure for these features is in place, waiting for hardware to control.

Back to that soon.

Re-grouping – QSD Receiver Project

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I built up the 8-pole audio lowpass filter board and it’s waiting to be tested. I also re-worked some of the DC power distribution so the whole receiver up to the audio power amp can be operated seamlessly on either 5V USB power from the Nano (w/no audio power amp) or on 6V-14V external power (w/audio power amp) without any switching.

In the process of doing that I blew up the Nano board and the TFT display board. I have the system back up and running, but with a junkbox TFT display board that doesn’t have touch capability. I have some new display boards on order, but they might take a couple weeks to arrive. Meanwhile, I’m cleaning up some firmware matters. I now have a function to save the whole radio state to EEPROM and automatically restore it on power-up. I also have some GUI cleanup matters that I’ve been putting off.

After repairing my EFHW wire antenna which was knocked down by a concrete truck when I had some work done on my house last year, I was a little disappointed to hear a local 1440 kHz AM station punching through down near the noise threshold. That station runs 5000 watts  and is only about 1/2 mile away, so maybe it’s to be expected. It does point to the need for front-end filtering.

Hopefully I’ll get the 8-pole lowpass installed and tested in the next several days.

Active Lowpass Filter and AGC Noodling – QSD Receiver

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Now that I have the signal chain functioning from antenna input to speaker output, I’m starting to work on the final missing function block – the steep-skirted audio lowpass filter. One of the PCBs I developed is an 8-pole active lowpass filter.

The 8-poie lowpass filter PCB

This will go just before the audio power amp which is presently an EBay LM386 module. I have a PCB for a TDA7052A which is an audio power amp with 80dB of voltage controlled gain reduction.

The TDA7052A Audio Amp PCB

One of the things I want to try is sampling the audio amplitude with a Nano A-D input and a software peak detector and using that number to index through a lookup table which gives a control voltage value to keep the final audio output constant. This “feedforward” method, if I can make it work. has several advantages which I’ve written about.

I hope to have the lowpass filter built up and in the signal chain this week.

Stay tuned

Joe

Gain Re-Distribution – QSD Receiver

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I went back to the drawing board to avoid what could have been a train wreck. My original design premise was to keep the amount of gain before the steep active lowpass filters as low as possible in order to prevent strong off-channel signals from overloading the subsequent high-gain stages. I neglected to model the noise performance of the unity-gain phase-shift network and associated active filter stages, thinking the 30+ dB of gain in the QSD post-amp would get the signal strong enough that subsequent noise wouldn’t be an issue. When I did model it late in the game, it was a revelation! So I went back and re-purposed one of the pre-filter stages immediately following the phase-shift/lowpass network to turn the last stage into an amplifier with a voltage gain of about 10x. (“20 dB” if you misuse “dB” as is done in common parlance). Later, I want to explore lowering the QSD pre-amp gain and raising the gain later in the chain. It’s a trade off between best sensitivity and best overall dynamic range.

I haven’t measured the audio output S/N ratio, but my ear test says it’s about 10dB at 1.0 uV or better. I’m going to go with that for now.

I posted the modified schematic on the PSN board. You can see it here.

Next up is the final steep lowpass network that will set the overall bandwidth to about 2.5 kHz with pretty sharp roll off.

Noise ! – QSD Receiver

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This morning while driving to the office, I had a frightening realization. The post amplifiers at the I & Q outputs of the QSD are a nice low noise design with an intended 30 dB gain. It’s closer to 40dB at the moment, but I really don’t want it that high in the long run. Without analyzing carefully, I’ve been assuming that much low noise gain would get weak signals (~ 0.5uV) up to an amplitude where I didn’t have to be concerned about the noise contribution of succeeding stages. Wrong!

The audio phase shift network (PSN), and especially the subsequent proposed active audio filters all have relatively large value resistors in the signal path. Each of those resistors is a noise generator. When I did an LTSpice noise analysis today, it became frighteningly obvious that I was headed for disaster. I now need to go back and re-scale the R and C values, and possibly adjust the gain distribution, to fix that. My major concern is the proposed audio lowpass filter design, since preliminary testing with my oscilloscope indicated that the sensitivity up to and through the phase-shift network is acceptable. However, that measurement was quick and dirty and was limited by how low my scope could read, so I’m not so sure now.

To verify that, I want to build a low noise test amplifier to boost the PSN output up to the point where I can more accurately measure the S/N ratio on the scope. With a test amp that can drive 50 ohms, I can also measure true audio S/N with my Textronix 495P spectrum analyzer, as that instrument can measure down to below 100 hZ with 10 hZ resolution. That will tell me how much sensitivity I’m actually losing in the PSN network, and what I need to do going forward in order to not degrade it much more.

Two steps forward and one step back. Typical.

QSD Receiver – Audio Phase Shift Network Added

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Today I added the audio phase-shift / lowpass filter board and did a quick sideband suppression test. It looks like 45-50 dB across the audio band, which is close to what the LTSpice analysis predicts. The receiver is now a single signal receiver; ie, a direct conversion receiver with a suppressed image response..

After the present lash-up is tested, I’ll add more audio lowpass filtering and an audio inter-stage amplifier. Gain distribution will be an important issue as I proceed from here. Too much gain at the wrong places and strong signal handling will be affected. Not enough gain  and the sensitivity will suffer. I’ll write about this as I proceed.

I’ll report more details as I get time to work on it.

Click Here for design details on the phase-shift / audio filter board

Click Here for details on all the boards

 

 

QSD Receiver – Antenna to Speaker Test

I got my replacement LT6231 op-amps and installed them on the QSD board, but only after mangling the fragile pads and traces. I had to install about 5 ‘fly wires’ using thin wire-wrap wire. I put a glob of hot melt glue over the part and its fragile connections to make it less susceptible to damage.

I had a little EBay LM386 amplifier module laying around, so I decided to put everything together on a piece of copper-clad board and do a little antenna-to-speaker test. No phasing network, no audio filter, no audio post detection filter yet. Still, it receives a clear audio tone with a 3uv signal from my HP8640B signal generator.

Next step is the audio phase-shift module, which will make this a single signal receiver. Then an active lowpass filter and about 20db to 30db more gain before the audio power amplifier.

Click Here to see the project from the beginning.