As promised, some detail about the subharmonic I/Q-SDR receiver. The RX has got a preliminary front end, just for testing, therefore not worth showing. Anyway, some pre-amplification is not that hard to design.
The trick with this design is, to shift the phase by exactly 45 degrees. This is accomplished by the RC combination. Theoretically, a phase shift of 45 degrees is reached when the condition R=XC is fulfilled. I chose a 1k resistor, thus, the capacitor should be 15.6994...pF 32pF (see comments below). Ah well, let's add a trimmer...
For the I/Q-stuff functioning, the levels of both LOs need to be exactly the same, therefore, I added a voltage dividing trimmer to the In-phase path. This trimmer is 2k in order to balance the RC phase shifter.
The total level is adjusted by the trimmer between the oscillator and the filter crystal. This trimmer seems to have a tiny influence on the phase shift...
So, here you got it, the first sneak preview of the 30m I/Q-SDR stuff. See my previous posting for other possible frequencies, most will not have the convenience of a canned oscillator however.
Hi Joachim,
ReplyDeleteI am sure you mean 90 degrees phase shift between I and Q and not 45 degrees. BTW, what is the idea of having to xtals? Are they in sync?
Andreas (dl8oam)
Hi Andreas,
ReplyDeleteno, I do mean 45 degrees phase shift at the generated frequency for the following reason:
I/Q-SDR requires 90 degrees phase-shift for the quadrature, but on the beat-frequency; that's what your comment is about.
The trick with sub-harmonic mixers is however, that those require signal at half (!) the beat-frequency. The mixer itself doubles the frequency. Doubling frequency also means doubling the phase-difference, hence, the phase difference at half the beat-frequency needs to be 45 degrees.
Therefore, the generated 5.0688MHz signal is split, one path phase shifted by 45 degrees, which at the mixers results in 10.1376MHz signals at the mixers, which are phase shift by 90 degrees.
73, Joachim (pa1gsj/dl1gsj)
PS: Forgot about your other question. This circuit employs one oscillator and one crystal. The oscillator generates a square-wave-ish signal, lots and lots of harmonics. For the sub-harmonic mixers to work best, a pure sine is required. And this is the job of the crystal, it is used as a crystal filter to get rid of all the oscillator's harmonics and form a signal close to a sine wave. One could do that with capacitors and coils.... many more parts would be the result.
ReplyDeleteI posted a sub-harmonic mixer circuit here:
ReplyDeletehttp://code.google.com/p/lemontree/downloads/list
Sean,
ReplyDeleteinteresting document. It will certainly give nice results. As I see it, it is a push pull configuration at the RF end, i.e. 180 degrees of phase shift. Note, it may look similar but will function in a totally different way.
I figure, your design could improve my 30m grabber receiver (http://qsl.net/dl1gsj/html/qrssrx30.html)
73!
Nice receiver. A similar circuit is TinySDR (www.qrz.lt/ly1gp/SDR/), which also uses subharmonic Polyakov mixers and phase shifting, though I must admit the phasing arrangement of TinySDR has me scratching my head (90 degrees at the LO plus an additional phase shift at the RF - perhaps 45 degrees? - with a final software-based AF phase shift).
ReplyDeleteI'm thinking a fixed-frequency, crystal-controlled DC rx like this might be a good IF stage for a simple single-signal superhet.
Hi qrp-gaijin,
ReplyDeleteI know the TinySDR circuit... I must admit that I don't understand the phase shifting either. I believe, you are right, it seems that the all important quadrature shifting takes place in the RF section.
Concerning you superhet idea, I was actually thinking along the same lines, e.g. for a traditional 9MHz i.f., 4.5MHz crystal are cheaply available. In this way rigs like the Drake TR4 can be equipped with a panoramic display...
73!
I am not clear about the termination impedance your mixers are seeing. As I understand it proper termination of the subharonic mixer with a diplexer is important for good performance (see e.g. http://www.noding.com/la8ak/images/RA3AAE-nix.gif and http://www.noding.com/la8ak/c21.htm). Do you have any thoughts on this?
ReplyDeleteThe design is derived from my QRSS-grabber receiver http://www.qsl.net/dl1gsj/html/qrssrx30.html which itself is based on PA2OHH's receiver design http://www.qsl.net/pa2ohh/09qrx1.htm
ReplyDeleteYour calculation for R=Xc, with R=1K and F=5.0688 MHz should result the value of capacitor 31.4 pF. Did I miss something? Thanks.
ReplyDeleteYou're absolutely right. For whatever reason I missed that factor 2.
ReplyDeleteMy RX, the one with the trimmer, works. I hope your's will work too.
I have not built one, yet. I will try to get one built soon. Another suggestion: Would it be more stable to generate 45 degree shift using a fix NPO capacitor at 32 pF and use a trimpot (1K value)with the center grounded? Using a VOM we can roughly adjust the value of the resistor 494 ohms to match the capactor's Xc. Just a thought.
ReplyDeleteI made a wrong calculation :-). For F=5.0688 MHz, and fixed NPO cap 32 pF. The Xc is 981.2 Ohm. And the trimpot should be 2K, adjusted with VOM to get to 981 ohm.
ReplyDelete