Simple 80m Grabber Receiver Ready

Next time I will visit the OZ9QV grabber "Thor's Hammer" there will be another band active in grabbing : 80m

I have finished encasing a modified RockMite 80 (RX only)

Frequency drift is acceptable :
10 deg C : 3499.955
23 deg C : 3499.915

Yes - negative temp coefficient.

Since the temperature is expected to be within that range (most likely 15 - 25 for the majority of the time, and I expect to put the RX and PC into a cupboard, I would expect the local oscillator to stay within 3499.925 +/- 10 Hz for most of the time, quite acceptable.

The addition of another band to the grabber is worth it alone, I think.
New antenna needed as well.

I know that more activity is on 40m by now, but the winter should be good for 80m experiments.

The grabber is expected to run with an older, low powered PC, together with the 10 MHz grabber.

The new grabber receivers should be installed before the end of the year.

I am playing with an idea for a more stable grabber receiver for 3500/7000/14000 kHz, more on that later.
It will be a bit more complicated to build, so I will be taking my time.

RockMite 30/80 Update

I have run a simple test of temperature drift of the two RockMites :

Using a fan blowing on the PCBs lying on the desk the RM80 drifts less than 10Hz when the fan comes on or off for about 5 minutes, that is quite acceptable, given that the RMs will be boxed, and likely I will use some insulating Styrofoam.

The RM30 is a bit more tricky. I tried to lower the frequency with a choke in series with the tuning trimmer capacitor I mounted, and the drift with fan was about 50Hz at on/off.

It turns out that grounding one side of the crystal resulted in a frequency of 10138.490. Not ideal for WSPR, but should be useable with WSPR2 and "BFO offset". Not tested yet, but will be in the near future. Temperature drift at the fan on/off is now 20Hz, and with a bit of insulation that should result in an acceptable temperature stability.

It looks like the drift is more due to the capacitors in the oscillator than the crystal in itself.

Oh, and I am a bit disappointed that the RMs have several zener diodes, but the oscillator itself runs from the raw supply voltage. Where did that idea come from ? I find it surprising that there is not more stability problems with the LO in the RMs

Next step with the RockMites : input crystal filter tuning - if necessary.

RM80 update

I have been operating the soldering iron and modified the RockMite 80 for 3500kHz.

The kit will be used as receiver only, so I bypassed the crystal tuning, ending up on a LO frequency around 3499.920, nicely suited for a grabber on 3500.800kHz

The receiver is probably not too sensitive, so I intend to test it, first with my 30m tuned loop, then most likely with an active antenna of some variety.

The first test has been monitoring the drift of the LO, it looks fairly stable, so a later test will be the rx sensitivity.

Next on the workbench will be the 80m Warbler, usable for PSK31 (of course) and a grabber on the colour burst frequency. I will be building it exactly as intended in the original design, then later build another for use on 40m WSPR.

Some experiments on the (V)XO of the RM80

After Joachim's experiments with the RM80 frequencies and MEPT, using the varicap, giving some problems with chirp I decided to try another approach, and we will see what the results are.

In place of the varicap I mounted a trimmer capacitor, in parallel with the R10 (feeding the varicap through I mount a 2.2nF capacitor, so that Q2 creates a RF-short of the trimmer capacitor, giving rise to the lower frequency RX LO of about 3579.350.

Just trying, and as expected the oscillator does not oscillate on the stray capacitance of the circuit without any capacitor in stead of the varicap.

First test was with a 2-10pF trimmer, giving rise to a frequency above 3580 (even with full capacitance), and stopping oscillation at low cap. values. Not good enough.

A 2-18 trimmer with a parallel fixed capacitor of 10pF is better : the (high) frequency range is now about 3579.75 - 3580.10. I noticed that the oscillation on the higher frequency is weaker on the higher frequency by about 10 dB, still the oscillation holds. A lower value coupling capacitor to the buffer, as Joachim indicates, is probably in order.

This makes me think that it should be possible to replace the trimmer with another varicap with lower capacitance than the one supplied., and with a zener stabilized voltage make the tx frequency tunable. A range of about 250 Hz should be attainable with one or two BB105 diodes - and I should have some of those somewhere.

On with the experiments, I will leave the RM80 with the trimmer cap for a while and see if the TX frequency remains stable when transmitting.

Update :

The lower value coupling capacitor to the buffer increases the oscillator frequency by 50 - 100 Hz - the oscillator now tunes down to .830 .
Parallel capacitor changed to 22pF, trimmer still 2-18pF, tunes (upper frequency) about .760 - just above 3580.000. nice range.