EMRFD Message Archive 6364

Message Date From Subject
6364 2011-06-04 07:08:32 wb8yyy_curt VFO Spot Frequenncy Error
I blew the dust off an old transmitter, my first QRP rig called the Ugly Weekender. This VFO is decoupled by a small series cap into the switch circuit to move the VFO when in the receive mode. When transmitting with the key it moves down as expected, and does the same with the spot switch -- except when the spot is used the VFO lands 800 Hz higher.

Does this imply there is some extra capacitance in the keying circuit? I guess of course! Is there a practical way to compensate for this, or am I going to have to learn to move the VFO by 800 Hz after using the spot switch? I may also examine to see if the 'move the VFO' is a function I actually need also.

I did discover I hadn't originally built the VFO with NPO's - the silver mica caps drifted a few kHz after turn-on - as most everyone here knows. Its stable enough now. Rig is plugged into a K8IQY 'magic box' and I am working on a homebrew version of hybrid cascode AGC to go into the receiver.

73 Curt
6365 2011-06-04 08:36:30 Paul Re: VFO Spot Frequenncy Error
If yours matches the '81 QST schematic, then the offset for spot and xmit are both achieved by switching in an extra 2.7pF into the VFO.

For both spot and xmit, this is achieved through transistor switch Q4.

The only difference I see is that the spot switch directly grounds the 10K base resistor for Q4, while the key does it via diode D3 (this is necessary to keep the spot switch from firing up the driver).

You could make this more symmetric by using another diode (like D3) in series with the spot switch (though I doubt this is it).

Maybe the VFO supply voltage is drooping on transmit. Is your power supply stiff? Measure the supply voltage for receive vs xmit. Maybe use a 7809 regulator for the VFO instead of relying
6366 2011-06-04 09:05:39 Nick Kennedy Re: VFO Spot Frequenncy Error
My recollection is that the Ugly Weekender VFO's shift was not something
like 500 to 800 Hz for a T/R function, but more like several kHz to get the
VFO out of the way of a separate receiver which had its own LO.

As far as the difference between spot switch and key - maybe by adding a
diode in series with the spot switch you'd wind up in the same place.


Nick, WA5BDU

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6367 2011-06-04 11:27:14 Wes Re: VFO Spot Frequenncy Error
Hi Curt and gang,

The typical shift was a few kHz downward when the switch was turned on. The value is not critical, for the VFO is not used during key-up periods. It was allowed to run to avoid warm-up drift that comes from turning the vfo completely off when not in use. In a properly operating version of the Ugly Weekender, there is very little difference between the frequency during spot activation and when the key is pressed. I measured about a 50 Hz difference.

You quoted a difference of 800 Hz. I would be willing to bet that you did the measurement without the lid on either the VFO or the PA, or perhaps both. Without the lids in place, energy from the PA can sneak into the VFO where it can alter the phase of the signal in the tank. A change in phase means a change in frequency. As I recall, we saw a difference of a couple of kHz shift when we ran the original box with the lids removed. This is all easy stuff to measure.

There are two solutions. One is to just put the lids on the two, or shield them in whatever way you elect. The preferred solution is to move the VFO from 7 down to 3.5 MHz and to then use a frequency doubler. With the VFO operating at 3.5, it is insensitive to energy that is outside the bandwidth of the oscillator tank, and 7 MHz is far removed from that. In you recall, the version of the UW that is in EMRFD uses a frequency doubler and is completely un-shielded. There is no detectable difference there between spot and key down.

Incidentally, the original UW (in transceiver form) is still around and kicking around Roger's house somewhere. Todd, VE7BPO, recently put a photo of it up on his web site, www.qrp.pops.net. Todd's site features ugly constructi
6368 2011-06-04 12:21:37 wb8yyy_curt Re: VFO Spot Frequenncy Error

Thanks for providing the data! Our frequency 'shift' is about the same.

Yes I removed the mods to support a DC receiver so this one has the large frequency shift when key up.

I built mine a few months after the article was published, but only had one box so hence there is no shielding. As is the keying sounds ok into a receiver, I think its mainly an inconvenience of the spot not being accurate.

So I will contemplate my options - either to 'halve' the VFO (yes I was curious about the new UW Xmtr) or remote the PA. I definitely want to preserve and use the rig. But I think I will get back to the receiver project first.

73 Curt

6372 2011-06-06 07:13:57 wb8yyy_curt Re: VFO Spot Frequenncy Error
When I posted I was seeking others experience on effectiveness of a VFO spot circuit without transmitting. Wes communicated with appropriate construction it can agree very closely with the transmit frequency .... so I started the experimental effort toward getting there!

Update: examining the board I saw that I have the PA and matching network condensed into a small area in the corner near the BNC output. so moving just a couple parts I added a 'vertical partial shield' made of double sided PC board. it completely covers the PA coils. I thought it might reduce the shift some. It actually increased it to around 1 kHz. I tried placing fingers in various spots to see if this shift would move - nada effect.

I do wonder if there could be cross coupling of transmit into the bias wire feeding the oscillator? I probably should examine this - and add some local capacitive and/or RF choke decoupling as an experiment.

I could reconstruct the PA into a smaller 'inner enclosure' but I have this pessimism that more might be lurking. Instead I might be more motivated to build a different PA circuit instead, with a little more output. A possible 'architecture' is to use the existing VFO (it is working too well to dismantle) with a new PA/keying circuit that is external. For this project I am not motivated to make this rig portable -- just use it as part of a homebrew home station (that does not exist in my shack with its K2, a few diverse QRP kits, and even my novice almost vintage rig etc.)

My receiver is ok except I really do not its audio amp -- its unstable as I move the gain control so perhaps my effort with SPICE wasn't so successful. I installed the 4.9 MHz LC matching network that I modeled onto the hybrid cascode 'ugly' board so its ready for test 'when appropriate.' I have the receiver boards (some kinda manhattan) mounted on a wooden board for ease of evaluation. Current architecture is mixer, two parallel NPN stages for a small linearity boost (I am told this circuit might be in EMRFD someplace but I haven't found it yet!), a fixed BW 4915 xtal filter and two stage IF similar to K8IQY 2n2-40, a product detector and my own audio amp.

I plan to work some more on this stuff when 6m Es dies down - I have a two element PVC/wire quad up in the air and it sometimes catches signals from Europe (2 CW QSO's so far).

73 Curt