Cleaning up the oscillator signal
The previous version of my homebrew RTL-SDR HF upconverter used a Pierce crystal oscillator with a crystal that was marked as 49.8 MHz.
Monitoring on a nearby receiver revealed that the oscillator was generating signal energy at 16.6 MHz and 33.2 MHz as well as at 49.8 MHz. Therefore, the simple Pierce oscillator was exciting the crystal at its fundamental frequency, with the desired LO energy at 49.8 MHz being merely a side-effect. So the LO was producing signal energy at many harmonically-related frequencies, which will introduce a host of unwanted mixing products at the mixer output, greatly increasing spurious signals and IMD.
To force the oscillator to oscillate only at the desired overtone frequency, I changed the oscillator topology to a common-base Colpitts, using the crystal to ground the base at RF. An LC tank circuit in the collector is tuned to approximately 49.8 MHz to prevent gain all other frequencies, in particular the fundamental frequency of the crystal. In practice, the base was first grounded with a 100 nF capacitor and the LC circuit tuned (by removing/stretching/squashing the turns on L1) to approximately 49.8 MHz. The oscillator's frequency could be monitored by watching the waterfall display on the gqrx SDR software (or alternatively by listening on a separate VHF receiver for the radiated signal). After adjusting L1 for oscillation at approximately 49.8 MHz, then the grounding base capacitor was replaced with the 49.8 MHz crystal. Finally, monitoring on a nearby receiver revealed an oscillator signal only at 49.8 MHz, but not at 16.6 MHz or 33.2 MHz -- just as desired.
The current circuit diagram is as follows.
I then connected the oscillator to the RF input port of the mixer, through a 100 nF capacitor. Unfortunately, this stopped oscillation. A 1000 pF capacitor allowed oscillation and was used.
Reception results (using my M0AYF-designed active loop antenna -- reference: http://www.qsl.net/m0ayf/active-loop-receiving-antenna.html) were much better than before with far fewer spurious signals. Shortwave signals were where they were supposed to be (WWV at 5, 10, and 15 MHz, ham signals at 7 and 14 MHz). Furthermore, occasionally ionosonde signals could be seen, and the ionosonde signal progressed from low frequency to high frequency on the software's waterfall display. If many spurious mixing products were present, we would expect mirror images or duplicate images of the ionosonde signal, but none were observed, indicating the HF upconverter is mostly working.
Here is a short video showing the reception results. You can see ionosonde signals at 01:35 and 02:33 in the video.
The video also shows the effect of adjusting the LNA gain, starting at 02:20. If the gain is too high, spurious signals and IMD start to appear. If the gain is too low, sensitivity suffers.
Here is a short video showing the physical layout of the completed circuit.
Next stepsThe crystal oscillator signal seems to be noisy, as evidenced by raspy tones when listening to CW signals. I suspect the 5V voltage taken from the USB hub is not sufficiently filtered for oscillator use. I will investigate voltage regulation and/or filtering on the Vcc line.
I still have no way of measuring the oscillator output power (e.g. an RF probe). Most likely, the oscillator is not delivering the required 7 dBm into the mixer's LO port. Nevertheless, the converter seems to be working reasonably well, so perhaps the LO drive is sufficient for casual listening purposes. If possible, I would prefer to keep the circuit simple (just 1 transistor) rather than adding more amplifier/buffer stages after the LO.
Some minor FM broadcast interference was still audible at some locations in the shortwave band, but the interference is limited to a few specific frequencies. A simple LPF on the RF port could fix this.
After playing with my upconverter a few days, I have a few additions and corrections to this article.
Correction: The LO is not noisy
I previously mentioned that the crystal oscillator seemed to be noisy due to raspy-sounding CW signals. It turns out that the oscillator is fine and is generating a clean signal.
The debugging process was first to listen to the radiated LO signal on a nearby receiver in CW mode. The radiated signal sounded very clean, which was puzzling because I suspected a dirty LO signal. Nevertheless, I thought it might be remotely possible that while the radiated signal was clean, perhaps the signal tapped off of the emitter (and fed into the mixer) might be dirty or distorted. The next step was to power the LO from a battery instead of from the USB power, to see if a dirty supply voltage was distorting the LO signal. However, powering the LO from a battery yielded no change in the quality of CW signals. Next, I powered the active antenna from a battery as well, instead of from an AC adapter. That also yielded no change. Therefore, either (1) the problem was due to noise somehow getting into the oscillator via another route, or (2) the problem had nothing to do with the oscillator. It turns out that (2) was the case.
The raspy tones I heard when listening to CW were due to a too-high sample rate being set in the gqrx SDR software. I was originally using a sample rate of 2.8 million samples per second. I reduced this to 2.4 million samples per second, and then the CW signals sounded pure and clean as they should. Probably, with the higher sample rate of 2.8 MS/s some samples were getting lost, which led to the unclean-sounding CW signals.
Perhaps the LO drive is sufficient
The web page of VK6FH at http://www.vk6fh.com/vk6fh/RF%20MIXERSdiode.htm describes some of the consequences of reduced LO drive level for diode ring mixers. A quote from that page follows, describing the effect of lower LO drive for commercially-available diode ring mixers.
Some designers shy away from using these passive double balanced mixers (DBM) because of the fairly high LO drive required. However, they can be used successfully with lower drive levels. The efffect will be the conversion loss and port-to-port isolation (defined below) will get worse, but since we are using these devices at the very bottom of their 1GHz range, the effects are minimal. LEVEL 7 mixers can be used by driving the LO port with a single transistor crystal oscillator, delivering about +3dBm which is 0.32V rms into 50 ohms (2mW ) with good results.VK6FH's web page presents a table showing the effects of reducing the LO drive from +7 dBm to +3 dBm -- slightly more conversion loss, and slightly worse port-port isolation. But these effects are not catastrophic, especially for a casual hobbyist receiver as this one is.
Also note that VK6FH specifically says that a single-transistor crystal oscillator (such as the one I am using) is capable of driving a level-7 mixer as long as the oscillator can provide about 0.32V RMS into 50 ohms.
So the question remains: is my single-transistor oscillator capable of delivering 0.32V RMS into 50 ohms? I lack the test equipment to test this currently. However, before building the oscillator, I did simulate it in LTspice and adjusted the circuit constants such that the simulated oscillator delivered more than 0.32V RMS into 50 ohms. Given the simulation results, and given the good reception results in practice, I see no need to attempt to increase the LO output (which would undesirably increase the circuit complexity).
A future article will explain the LTspice simulation of the local oscillator circuit.