++++++++++++++++++++++++++++++++++++++++++++ Date: Mon, 23 Apr 2001 17:07:58 -0700 From: Wayne Burdick Subject: [Elecraft] Re: How to fix the FREQUENCY SHIFT between T/R ja9mat (h.komachi) wrote: > I found that the transmit frequency is different from the > receiving. The difference is about 10Hz order. > PSK31 needs high accuracy you know.... Mr. Komachi, The next revision of the K2 manual will deal with this situation in more detail. Here is the information as we have it at present. It will be posted on our web site along with other data-mode application notes. 73, Wayne N6KR * * * Using the K2 on PSK31 and similar modes If you have the KSB2 SSB Adapter installed, you can operate PSK31 mode. The easiest way to get on PSK31 is to configure FL1 as OP1 using CAL FIL, then use only FL1. The transmit and receive frequencies will then be identical as long as you don't use SPLIT, RIT, or XIT, and this will keep the software tracking properly. If QRM is a problem during PSK31 operation, you may wish to switch to the CW filter for receive. You can use FL2-FL4 for this purpose, setting up filters from about 150 to 1500 Hz wide. However, when you switch to one of these filters, your transmit and receive frequencies may now differ by a small amount (typically 10-20 Hz). The reason for this is that the K2 uses two independent crystal oscillators, one for the VFO and one for the BFO. The VFO frequency is adjusted in firmware to compensate for the different BFO settings. But the resolution of adjustment on both oscillators is only about 10 Hz, not 1 Hz. So, even with careful adjustment of your BFOs in CAL FIL, it is not possible to obtain better than 10 Hz of T/R offset. During PSK31 QSOs, this offset can cause the software at both stations to "walk" up or down the band slightly as it tries to match the transmit and receive pitch. To avoid this problem, you can: (1) use FL1 only, as described above; the RX and TX frequencies will be identical as long as splits are not used (2) turn your software's "netting" feature OFF (it may also be called "AFC", etc.); this prevents the software from attempting to pitch-match, so that when you change filters, it will have no effect on your transmit center frequency (3) some PSK31 software may provide a way to compensate for T/R offset by manually entering an offset value (this is a very new development-- contact the software suppliers for further information) Note: A future version of the K2 firmware will probably provide a fourth option, FINE RIT mode, which will allow you to manually adjust the receive offset of one narrow filter with respect to FL1 (as long as FL1 is still set for OP1). You can typically get RX/TX frequencies matched to within about 3-5 Hz using this method. However, you may still need to turn off netting in your software, depending on how much receive pitch shift it can accommodate. ++++++++++++++++++++++++++++++++++++++++++++++++ Date: Mon, 23 Apr 2001 20:25:22 -0400 From: Charles Greene Subject: Re: [Elecraft] How to fix the FREQUENCY SHIFT between T/R At 06:43 AM 4/24/2001 +0900, Hidehiko KOMACHI wrote: >Hi fellows, > >Lastweek I enjyoyed PSK31 using K2. >I worked several station including overseas DU,UA3,UA9 and YC. >But I found that the transmit frequency is different from the >receiving. The difference is about 10Hz order. > >PSK31 needs high accuracy you know.... > >I'd like to try to fix this frewuency shift. >But I'm not sure how I adjust it. > >Some fellow QRV on PSK31 and does anyone has the same >problem? > >Please let me know your suggestion.... 72! > >ja9mat Hi, There is a frequency offset between the filters OP1 and FL2, OP1 and FL3, and OP1 and FL4. If you narrow the bandwidth of FL1, there is a difference between OP1 and FL1 too. Since OP1 is always used for transmit in SSB, there will be an offset between TX and RX if you are using any filter for receive except OP1. You are correct in that the frequency offset is about 10 Hz minimum. You can use the program Spectrogram and receive a constant carrier (the birdie near 7 MHz will do) and measure the frequency difference between the carrier received using OP1, FL2, FL3, and FL4. The difference may be greater than 10 Hz, but careful alignment of the BFO can get it down to 0 to 10 Hz. I tried to center the tone in all filters and that seemed to help. If you only use OP1 as a receive filter, you don't have the problem, and for starters, I recommend that as a normal mode of operation. If you need a narrow filter because of QRM, there is a work around: Once you have made contact with a station and you are using a narrow filter, turn "net" off, or "lock" your transmitter frequency, depending on what your program lets you do. (Digipan and MixW32 both use "lock," and all of the others that I am aware of use "net.") That means you are receiving and transmitting on slightly different frequencies, but as long as both you and the other station's frequencies are reasonably stable, there is not a problem with this mode of operation. In fact, the recommended way to operate PSK31 is for one of the stations to turn "net" off. Since there is no convention as to which station turns net off, usually neither station bothers to do it. So turning "net" off is a preferred mode of operation anyway. There may be some information of the Elecraft web site giving some advise too, so you can check out that out. 73, Chas, W1CG +++++++++++++++++++++++++++++++++++++++++++++++++++++++++