TheRadioBoard

Greetings,

I'd like to request some advice on a DC receiver design I'm pondering. What do you think of the following? Goals are single-signal reception, continuous coverage 3-30 MHz, easy bandswitching, and easy tuning. Proposed architecture:

Front-end single-tuned LC bandpass filter, followed by RF splitter, feeding 2 Polyakov mixers. LC VFO runs 6-60 MHz, fed through f/4 flip flops to generate I/Q signals from 1.5-15 MHz (half-frequency I/Q VFO signals for Polyakov mixers which run at half-frequency). Follow mixers with diplexers, low-Z AF preamps, and op-amp-based AF phase shifters. Finally, sum the AF channels.

I believe the VFO arrangement would provide accurate, no-adjustment-needed I/Q signals from 1.5-15 MHz which could then switch the Polyakov mixers yielding 3-30 MHz reception. Tuning would require peaking bandpass and adjusting the LO - two knobs, but unlike a regen which also has 2 knobs for tuning, the DC tuning should be less critical and the performance also better (dynamic range, high opposite-sideband suppression). I am a little worried about the LC VFO stability at 60 MHz, but I can always replace it later with a DDS if the receiver proves itself worthy.

One thing I'm not sure about is low-pass filtering of the I/Q VFO signals: I understand they will be square waves. Do I need to low-pass filter these signals before feeding them to the Polyakov mixers?

Any comments or advice appreciated! Thanks in advance.

Like this?

http://www.arrl.org/files/file/Technolo ... 301032.pdf



quite an ambitious project!

73
kb0lxy

Hi Ham-er,

Thanks for the reply. The design you referenced looks great, but uses narrow-band phase shift networks at the LO to generate the I/Q signals, so the phase adjustment has to be adjusted if you change bands. I was thinking about using flip flops and a 4x VFO to generate I/Q signals with no adjustment needed - but for an up-to-30 MHz receiver, this requires an up-to-120 MHz VFO, likely very difficult to homebrew with adequate stability. That's why I was wondering about using Polyakov mixers, which only require a half-frequency VFO, to cut the maximum required VFO frequency to 60 MHz.

However, doing some more thinking about and investigation into Polyakov mixers reveals that for opposite-sideband cancellation, they require an I/Q phase shift of 45 degrees (maybe we can call it an "octiture signal" for 1/8th of a cycle ), not 90 degrees - which makes sense since the Polyakov mixers essentially multiply the VFO frequency by 2, so they also multiply any VFO phase shift by 2. But, the flip flop and f*4 method of generating I/Q signals produces a phase shift of 90 degrees, which I now believe can't be used with a Polyakov mixer. So we either have to find another way to generate the 45-degree-offset I/Q signals (like the aforementioned manually adjusted narrow-band phase shift networks), or use a "normal" mixer taking 90-degree-offset I/Q signals, but accept the requirement of an up-to-120 MHz VFO. A superhet-style arrangement is also possible with a fixed-frequency phasing DC detector, but I was hoping to avoid the addional spurious signal headaches that that would cause.

Any ideas on how to generate I/Q signals with 45 degree separation, or to generate 90 degree quadrature signals without requiring an f*4 VFO, or other creative ways to do opposite sideband cancellation with Polyakov mixers?

And about this being an "ambitious" project: it looks like phasing DC receivers consist of a large number of non-critical stages, unlike regens which consist of essentially a single, highly critical stage. With regens, you have to worry about input signal level, tank Q and loading, regeneration level, and detector operating point. On the other hand, it seems like with a phasing DC each of the small stages (LO, RF phase shifters, mixers, AF phase shifters) should be pretty easy and non-critical to build. I may revise my opinion as this project progresses .

There are probably a number of ways to achieve a 45 degree phase shift !

Digital delay (counters timers etc.).

But you seem to be asking for a simple way to achieve it, and to do so for any and all VFO frequencies.

Im sure it probably IS possible to build an image canceling DC receiver using polyakov type mixers but I dont beleive it would be simple or "non-critical"!

90 Degree phase shift is much easier.

It's been quite a while since I worked with "digital circuitry", so I could be wrong.

A complicated task can be done, but complicated tasks usually require complicated circuitry as a general rule.

Luck
kb0lxy

I think you're right about 90 degrees being easier - at least, it's easier to find information about. Quick summary of what I've found so far:

To create 45 degree phase shift we can:

1) use one series RC network at the VFO, between signal and ground, with capacitor at top. In-phase signal taken from top of network, "quadrature" (octiture?) signal taken from RC junction. R is set equal to the capacitive reactance. Covering different frequencies requires changing both the phase network (R and/or C) as well as an amplitude-balancing adjustment.

2) use two series RC networks, one at each branch of the split RF signal path. Both RC networks are adjusted for a total phase difference between the two signal paths of 45 degrees. Covering different frequencies requires tuning (possibly) both RC networks, and an amplitude-balancing adjustment. (EDIT: I don't think this option is needed in practice. Polyakov mixers double the LO frequency and LO phase shift, and hence need 45 degree phase shift at the LO. But Polyakov mixers don't double the RF, so we should use a 90 degree phase shift in the RF path with Polyakov mixers.)

3) use a 90 degree phase shift arrangement for the VFO (see below) plus a 45-degree RC phase shift for the RF signal. (I saw a design, TinySDR, that appears to do this; I hope I did not misinterpret the intention and function of the phase shift networks.)

4) other ways, perhaps using logic gates? I don't have any information on this at present.

To create 90 degrees phase shift we can:

1) use one RC series network at the VFO, connected across a transformer secondary winding, and with the RC junction grounded. Signals at opposite ends of the secondary winding will by definition be 90 degrees out of phase since the R and C are in series. Covering different frequencies requires only one adjustment: an amplitude-balancing adjustment of either R or C so that R is equal to (?) the capacitive reactance.

2) use 2 series RC networks in the RF signal paths, adjusted for a total of 90 degrees phase shift. As with the 45 degree case, covering different frequencies requires potentially changing both networks, and an amplitude correcton.

3) use flip flops and a f*4 VFO to generate I/Q signals over the entire VFO range with no phase adjustments required. Output will be square wave.

4) use a DDS to directly generate the I/Q signals.

5) other ways not yet researched...

I thought Polyakov mixers, given their good reputation, performance, simplicity, and interesting properties like only requiring a f/2 VFO, might simplify an all-band DC receiver design. But it seems that if we want a phasing DC receiver, the Polyakov mixer may actually complicate things due to the necessary 45 degree phase shift.

Corrections, additions, suggestions are welcome!

I see no advantage to Polyakov mixers for stability. They take an input LO signal and effectively double it using the dual diodes. So any frequency instability will also be doubled. In fact there's really no free lunch no matter what you use. You need a rock stable frequency source for your LO. I don't see this happening with a VFO except maybe for one designed to operate over a narrow frequency band (something like a Vackar circuit). To get stability, the only solution that I can see is a digital frequency generator of some kind.

There is a possible way to create I/Q LO signals from a 2xVFO if you have a differential VFO output to drive two flip-flops.

The downside is that if the duty cycle of the incoming clock is not 50% there will be some quadrature error that will need correction. Maybe this is what you mean by "requiring adjustment?"

Second ...
I agree with Bob that a synthesizer PLL maybe necessary to stabilize the VFO frequency.

Kevin

_________________
Überdyne: An extraordinary small force.
Kevin

About stability - my regen is pretty stable at 7 MHz, and even seems listenably stable at 28 MHz. So I'm thinking a VFO run at the signal frequency could be stable enough in practice. (f*2 and f*4 VFOs, probably not.) For the 90 degree phase shift, a 90 degree phase shift transformer would seem the best option - easy 90 degree phase shift over a wide frequency range with just one adjustment, the amplitude balance.

Now that Polyakov mixers are out of the picture, the design is starting to sound more like the R2 receiver linked to above.

A good idea for improving Direct Conversion receivers is using a separate regenerative stage (Q-Multiplier) around the input tank circuit. That will improve the sensivity and selectivity.

DrM,

Respectfully, I have to disagree with you. A properly designed and constructed direct conversion receiver will not benefit from a Q-multiplier on the input tank. Although signals may sound louder they will have a lower SNR due to undesired device noise and intermod products introduced by non-linear mixing occurring ahead of the product detector. DC receivers certainly don't need any help in the sensitivity department... even a simple Polyakov or DBM is inherently capable of -130dB sensitivity. They do benefit from a passive bandpass filter ahead of the mixer and a low-gain rf stage to reduce or eliminate potential LO feedthrough to the antenna. Above 20MHz or so a grounded gate preamp serves well to overcome mixing losses and set the overall system noise figure. Below 20MHz the rf stage serves primarily to isolate the LO signal.

73,

'Bear' NH7SR

I think the approach that I would take is to use the transformer & RC quadrature network. Then amplify & clip the quadrature signals. This will result in equal amplitude quadrature signals, which should work fine in the remaining part of the circuit. You could build a lot of the circuit with high speed CMOS logic. The only drawback to using squarewaves in the mixers is that it will produce mixer products of RF signals that are at the LO's harmonic frequencies. Decent preselection at the front end should eliminate this or at least reduce it to acceptable levels. Squarewave mixing shouldn't affect the remainder of the phasing circuitry because any other harmonic products would be well above the audio range and could be removed with a low pass filter, or simply ignored.

Note, you would still likely have to switch different capacitors into the RC quadrature network for different frequency bands, but the clipping technique would reduce the need for constant readjustment as you tune though a band.

YES
An RC network will phase shift a signal, but will only shift it to exactly 45 degrees at only 1 single frequency!

@QRPBear:


You can build a quick and dirty Direct Conversion receiver by using two regenerative receivers. One regen as the main receiver. This regen is set just below the threshold of oscillating. The other one is used as the BFO for clarifying th SSB signals. More info can be seen at topic "Quick and Dirty Direct Conversion Receiver" http://theradioboard.com/rb/viewtopic.php?t=3155

Schematic:

I was actually referring to this type of circuit:

(Change "Carrier In" to read "Local Oscillator In")

It will give a 90 degree phase shift at all frequencies, but while the outputs are always exactly 90 degrees apart, the output amplitudes will only be the same at one frequency. That's why I suggested an amplifier/clipper arrangement to restore the the amplitudes to a constant value.

The reason I recommended switching in different capacitors for different bands is because, as frequencies increase, the voltage across the capacitor will decrease and may eventually reach a point where the succeeding buffer/clipper stage doesn't have enough gain to restore the amplitude. In this case, a smaller capacitor is switched in.

Thanks Bob for the lucid explanation. You just answered some lingering questions I still had.

The mechanical considerations of switching in different capacitors seem a little unwieldy (for me anyway), so perhaps another way would be to use fixed R and C, and a pot to adjust the VFO level as needed to ensure clippping. Or, perhaps we could omit the clippers, keep the VFO level control, keep C fixed (10 pF maybe?), and make R variable for the amplitude balance. Variable R would be adjusted for maximum phase cancellation and VFO amplitude would be adjusted for optimum mixer injection (both judged by ear).

I do like the clipper idea, but lack the appropriate equipment to measure the RF waveforms to ensure clipping, so I'm not ready to jump in on that just yet. It's a great solution though: perfect I/Q signals for all frequencies with no adjustments and without requiring a f*4 or f*2 VFO.

By the way, when are the I/Q amplitudes equal? Is it when the resistance equals the capacitive reactance? (Mentally rearranging the transformer's secondary circuit so that R and C are connected to separate grounds, and recognizing that I and Q signals are taken off the "top" of R and C, respectively, makes it seem the amplitude-balanced condition must occur at R=Xc, but I just wanted to make sure.)