2128	2008-09-25 06:10:12	Heikki Ahola (OH...	Carrier oscillator drift
Message	Date	From	Subject
Hello group, After a lengthy period with no contribution to this groups from my side, I would like to introduce a problem that has been bothering me for some years already. My 100% homebrewed tx chain starts with a SSB generator on 10.050 MHz (some surplus xtals I had access to !!!)followed by conventional transverter stages, filters and amplifiers. The operation has been satisfactory in SSB and RTTY modes where frequency stability is not a big issue, what I mean is that a drift in the order of 10-20 Hz can be neglected. This is not the case in modes like PSK31 and I have tried to trace the source of this kinda drift. VFO is based on AD9951 DDS which should be OK, and this leads us to the crystal oscillators. What I suspect most is the carrier osc which is a VXO )T68-2 inductor and 50 pF var C) actually, this enables operation in various modes and with homemade ladder filters not perfectly matched with the carrier frequency. Question: Has anyone experience on the frequency drift of cerrier oscs and VXO's in particular? The drift occurs during a normal transmitting period of 3-5 min which suggests that some heating might take place. To improve the situation, I have already planned to construct a separate carrier oscillator for PSK31 only, although I may run into problems in finding the right crystal. Luckily in PSK31 I only need one sideband which facilitates the selecting procedure a bit ! BR Heikki Ahola (OH2LZI)
2129	2008-09-25 07:39:56	victorkoren	Re: Carrier oscillator drift
First you have to find the element that causes the frequency drift. If the problem is thermal the easiest way is to heat each suspected component by putting a soldering ir
2130	2008-09-25 20:23:03	Allison Parent	Re: Carrier oscillator drift

2131	2008-09-26 00:45:22	Heikki Ahola (OH...	Re: Carrier oscillator drift
Hello Alison and Victor and the rest, Tnx for the comments and advice! I realize now that also the DDS (actually two of them, one for the rx and another for tx might cause this kinda drift. The reason for having two of them is the two different if frequencies (10.05 for tx and 8.000 for rx). Since I am planning to replace the older AD9851 with AD9951 I might consider running them both from the same crystal oscillator (right now they have oscillators of their own). Although I have a new thing to suspect, I will need to study the carrier osc also and make some drift tests during the long dark winter nights, hihi. In addition, switching the sidebands during operation and monitoring the drift direction should provide some info on the origin. I am aware that software can compensate drift in the received signal, but the transmitted signal is not tracked. Since I am using completely separate rx and tx, both are free to drift whichever amount and amount they please, that seems to be my basic problem right now (if one considers an 20 Hz drift to be a problem at all !!!) As usual, I will keep on experimenting . 73 de Heikki (OH2LZI)
2132	2008-09-26 01:40:52	brainerd@wildblue...	Re: Carrier oscillator drift
On 26 Sep 2008 at 7:45, Heikki Ahola (OH2LZI) wrote: > Hello Alison and Victor and the rest, > > Tnx for the comments and advice! I realize now that also the DDS > (actually two of them, one for the rx and another for tx might cause > this kinda drift. The reason for having two of them is the two > different if frequencies (10.05 for tx and 8.000 for rx). Since I am > planning to replace the older AD9851 with AD9951 I might consider > running them both from the same crystal oscillator (right now they > have oscillators of their own). > > Although I have a new thing to suspect, I will need to study the > carrier osc also and make some drift tests during the long dark winter > nights, hihi. In addition, switching the sidebands during operation > and monitoring the drift direction should provide some info on the origin. > > I am aware that software can compensate drift in the received signal, > but the transmitted signal is not tracked. Since I am using completely > separate rx and tx, both are free to drift whichever amount and amount > they please, that seems to be my basic problem right now (if one > considers an 20 Hz drift to be a problem at all !!!) > > As usual, I will keep on experimenting . > > 73 de Heikki (OH2LZI) > You could use a VCXO with the DDS and lock the VCXO to a GPS receiver. Dave - WB6DHW <http://wb6dhw.com>
2133	2008-09-26 05:04:20	Allison Parent	Re: Carrier oscillator drift