Subharmonic 30m RX Design

30m QRSS Grabber Receiver

The basic design is a mix of ideas gathered from pa2ohh, the ARRL Handbook and publications made about the subharmonic mixer of Prof. Polyakov (ra3aae).
Since I am a really lazy dawg, thinking came first...

What we want to observe is 10140000 to 10140100Hz. Let's see what is available.... 10.140MHz crystals, close, but too close for comfort. Pull those crystals, and oscillators will become less stable. 10.125MHz crystals... too far off! 10.135MHz, closer, possible, but still pretty far off; 5kHz, oh my goodness!

But we also got really cheap and well available crystals and oscillators (!) at 5.0688MHz. Multiply by two and we are at 10.1376MHz, that's just 2.4kHz off the lowest frequency we want to receive. 2400-2500Hz also is in the comfort zone of any random soundcard at even low sampling rate.
To use this particular frequency, all we need to do is, multiply by two, or in other words, double it.

Jan (pa9qv/oz9qv) pointed me towards Prof. Polyakov's mixer, also known as the Russian mixer. Great readings from late la8ak, check this out: http://noding.com/la8ak/c21.htm

Thus, there we are, ultra-cheap 5.0688MHz CMOS-oscillators combined with a Russian mixer receives the 30m-qrss-band at about 2.4kHz. Let's design something!

Final design

A Polyakov mixer is supposed to work best with a sine signal. The CMOS-oscillator's signal however looks more like a squarewave, involving a lot of harmonics. The easiest and cheapest way, as it seemed to me, to transform that into a sine, is a crystal in series resonance.

The unwanted lower sideband has been taken care of by including a 10.140MHz-crystal behind the preamp.
Behind the filter, a RF-amp is added to compensate for losses. Due to this, the filter response is now shifted up again, I will look into this, but for now, I will let the receiver run and collect data/info.
As promised, I looked into it, shifting a crystal up is done by capacitive load. Reducing this load will bring the crystal closer to its series resonance frequency. To be a little bit on the adjustable side this time, I added a trimmer found in my junk box. I thought it would have a max. capacity of 18pF. This trimmer allows for adjustment of the crystal filter pass band frequency.

Consideration for operations w/o the filter: we are looking at a bandwidth of 100Hz and the received LSB will be (about) 10135100Hz to 10135200Hz. I figure, there is just the occasional Feldhell QSO, but other than that, the LSB range of the DSB receiver seems to be an empty spot.

A jumper is added in order to enable support of an active antenna via coax cable. The ideas for an active aerial are floating presently. It will be a shielded resonant loop with a preamp, that's for sure. The loop will be made from "YMVK-as 2x2.5mm²" which is underground twin-lead mains cable, doubly shielded. In an earlier experiment I successfully made a 40m-75m RX-loop from that stuff.

Subharmonic 30m direct-conversion receiver

 

Photo of the 30m direct conversion receiver

The actual receiver, as running for many years

How to tune the suburban sub-harmonic receiver

Some help to adjust the balance and drive of the sub-harmonic receiver used in my grabber (dashed lines indicate 0.7V cut-in voltage for Si-diodes, V is mixer drive voltage).

1) Mixer correctly driven: Within one period a single diode opens for exactly 90 degrees (red). Within the same period, the other diode opens for 90 degrees (blue), while the rest of the time, the diodes are both closed (yellow). As you see, the first diode opens at 45 degrees, that is very important! You also see, the entire mixer is open twice within one period; that’s the trick of the frequency doubling. Any off-balance shift will produce harmonics, which you can’t get rid of.

2) Let’s have a look at the mixer being extremely out of balance. In this example, -0.7V, which would open the second diode, is never reached, hence, this diode will never open. Consequently, the mixer will open once in a period, thereby not doubling the frequency. Still, due to under-sampling, some signal will be received, however, this is essentially a DC-RX for the LO-frequency. Actually, one can make use of this, e.g. for a 14MHz sub-harmonic receiver, which, when adding the correct off-set, will turn into a 7MHz DC-RX (either with a second crystal filter, or non at all).

3) And now, the mixer in balance, but over-driven. This will let a lot of signal through and will actually also double the frequency. However, the phase of the mixer is now totally off, i.e. asymmetric. This will produce a lot harmonics, so the receiver will receiver on many multiples of the local oscillator.

Actually, it takes a bit of patience to find the sweet spot in such a receiver. For me, it works well, for many years by now.

QSO?

What is a QSO nowadays? Not sure, in particular when talking QRSS.

The latest fashion seems to communicate via an online grabber. This this a QSO?
I don't think so, for the following reasons:

1. both stations have contact to the grabber, which is rather an automated SWL connected to the internet
2. there is no wireless link from the online grabber to neither of the two stations
3. the online grabber does not repeat a signal per se

However, all in all, I believe using a grabber to communicate is somewhat fun, since the communication is somewhat full-duplex.

What amateur radio is concerned, there still is the issue of dependency of many non-amateur communication channels, namely the ISPs of the three involved parties.

Small Wonder Labs closed

Today I learned that Dave K1SWL closed his QRP business for good. Very understandably, after many years of excellent service.

I would like to thank Dave for all the inspiration he brought to the QRP community over the decades and wish him a good time in the house he built in NH!

72 es 73

the old phase shift question

Actually I am rather flattered by the fact that an experimental design, which I did a couple of years ago, still is under discussion.
The good ole question about how much phase shift you need to run Polyakov mixers for I/Q software defined radio (SDR).
Have a look: http://draaggolf.blogspot.com/2010/01/30m-subharmonic-iq-sdr-receiver.html
Yep, this is a very simple concept design, it worked, it suppressed the lower side-band. And yes, it uses 45 degrees of phase shift at the oscillator frequency, which is half the beat-frequency (for reasons of using frequency doubling Polyakov mixers).
Please see the comments on why other designs, resulting in 180 degrees of beat-frequency shift, still seem to work just fine.
My design is not perfect at all, using a potentiometer to adjust a radio-frequency phase-shift is neither elegant, nor stable. Keep in mind, that this quick and dirty design was more a proof of concept rather than a production ready machine.
73!

12m XTAL Controlled QRSS / WSPR / QRP / SDR

In a previous post, I described the use of CD-crystals for 17m weak signal operations. This post is attempting to do the same but for the 12m.
The crystals I am mentioning here are also used in CB-rigs, although harder to find. Here's would be a possible source.

QRSS: 37.340MHz => 12.446MHz x 2 = 24.892MHz
QRP/SDR: 37.350MHz => 12.450MHz x 2 = 24.900MHz
WSPR/QRSS: 37.395MHz =>  12.465MHz x 2 = 24.930MHz

SW+ 40 possible WSPR/QRSS mod

Regular readers of my blog will know that a PSK-Warbler could easily be modified into a 40m WSPR transceiver. I believe that Dave K1SWL even mentioned this mod during his talk at the FDIM-QRP-Convention.
Very unfortunately, the Warbler has been retired later that year. It seems the days for easy 40m transceivers are over.

However, there is hope, the 40m SW+.

The trx employs a 4MHz crystal filter for reception, since the SW+ is a superhet A1A transceiver, there is no if-filter in the tx-train.
The transmitter consists of the VFO, and up-converter (NE612), a link-filter, linear buffer and driver and a class-C power amplifier.

This is what I see, could be done to the rig... please grab a circuit diagram (can be found on K1SWL's homepage).




Concept


We want to have zero-beat at 7038.6kHz. So the sum of intermediate frequency and local oscillator should end up there. With the 4MHz intermediate frequency the kits comes with, we would need a frequency source of either 3038.6kHz or 11038.6kHz. Both are somewhat hard to reach on standard crystals.
Now, lets have a look what get's us close...
Lets check out the options:

1. 5.000 + 2.048 = 7.048
2. 5.200 + 1.843 = 7.043

Option one will require penning down the 2.048MHz crystal, however would open the option of using a 2.000MHz crystal for the lower band edge QRSS range. Additionally, a 2MHz ceramic resonator oscillator would enable operation in the regular CW portion of the 40m band.
Option two would probably work by just severely pulling of a 1.843MHz crystal.



VFO


This is were the mod begins. The VFO should be skipped; an external xo will be doing this job. Having the local oscillator external will provide some advantages for QRSS operations. First of all, temperature stabilization should be applied. Secondly, one may consider switching between several crystals.
Most importantly, an extra 7dBm output should be added for a transmit DSB modulator.


BFO

In a side-band transceiver, an offset between the BFO and the up-converter is not wanted. In a CW transceiver, one wishes to have an offset of about 800Hz between RX and TX. Since the SW+ is a CW transceiver, we would like to get rid of the off-set. The easiest way of doing this would be to buffer and feed the BFO into the up-converter.


Receiver

The receiver is pretty much were we would like to have it. All crystals should be changed according to the mod you would like to perform.


Transmitter

This is were the choice of option will become important. Buffer and driver look just fine, however, it is definitely required to convert the PA from class C into class AB for linearity. It might be easiest to just design a linear PA, potentially external, and feed the buffer into it. Another option would be to merge designs. Genesisradio.com.au once sold a qrp-transmitter kit, the Q5, which was equipped with a 500mW class A linear PA. The PA design itself genesisradio took from their SDR-TRX.

Superhet crystals for QRP

This post is incomplete and will be updated in a later stage.



160m

• 1.843

80m

• 3.560 = 13.560 - 10.000
• 3.560 = 11.059 - 7.500 = 11.059 - (15.000/2)
• 3.566 = 8.000 - 4.4336
• 3.554 = 6.5536 - 3.000
• 3.690

60m

• 5.360 = 15.360 - 10.000

40m

• 7.03(0/4) = 17.734 - 10.700
• 7.090(0/6) = 4.096 + 3.000

30m

• 10.106
• 10.116

20m

• 14.060 = 11.059 + 3.000
• 14.285 = 9.216 + 5.0688
• 14.28(5/8) = 12.288 + 2.000

17m

• 18.086
• 18.096 = 14.000 + 4.096
• 18.096 = 16.000 + 2.097
• 18.106 = 10.106 + 8.000
• 18.130

15m

• 21.060 = 11.059 + 10.000
• 21.28(5/8) = 12.288 + 9.000

12m 

• 24.906
• 24.950

10m

• 28.060
• 28.365

6m

• 50.060
• 50.285

2m

• 144.060
• 144.285

Frequencies in MHz. Italics denote J3E frequencies.

new blog created

Dear ham radio operator,
Lately I filled this blog, my radio blog, with some computer / IT related posts. This was due to a lack of a dedicated place to publish those posts.
It seems inappropriate to continue with this practice, hence, I decided to create another blog concerned with this sort of technology:
http://homebrew-it.blogspot.com/
Hopefully I will have to report something radio in the near future.
73

ccrrrcrcrc crcrrcccrccc ... SILENCIUM!

My workstation employs a mouse which performance-wise, I really like. It is a super cheap Logitech M100. The only thing that is really annoying is the sound and cheap feel of the scroll wheel.
I know, this is a matter of taste. However, here's one to try for yourself (and this may apply, mutatis mutandis, to other computer mice too!).

The feel and sound is created by a spring mounted against the inner corrugated surface of the wheel (of course, first you need to remove the single screw on the bottom of the device... you know all about this....

overview of the mouse

close-up of the click device

Key point of this very simple mod, compared to other attempts available in the mists of the internet, is to remove the bloody click thing all together.

In order to achieve the goal, the only thing to do is gently slide out the scroll wheel assembly. There is nothing holding down said assembly, just grab it and off it goes. The photograph below shows the assembly, the clicky spring device still in place, whilst the load spring held on the axle just fell off (this spring we actually want to place into its original place when assembling the mouse again!).

the wheel assembly taken removed from the mouse, note the load spring

Now, gently pull the actual wheel from the assembly. The clicky-clacky (ccrrrcrcrc crcrrcccrccc) spring will most likely fall off in the process. Do to the severe complexity of this stage, I forgot to take a picture... sorry for that ;-)

The following step will be to put the wheel back in its original position in the assembly, w/o the crcrcr-spring of course.

As a last step, we slide the wheel assembly back by using its guard rails. Mind the "scroll wheel click load spring"!

now w/o the noise device

Again, I leave it all to you to put the lid on it all and screw it all down by the single mounting means we had to dismantle in the first place.
As a result, I feel personally very positive about the modification. Not only is the bloody noise gone, the now freely spinning scroll wheel provides a real smooth experience.