Peter, Wayne To add to the discussion of TX spurious signals:
I did perform extensive (spectrum) measurements on the TX spectrum at 28MHz and found that (despite the class A / High IMD behavior of the TX buffer Amp) some in band signals are generated in this amplifier. Example: TX = 29.0 MHz Spurious found (only -40 to -50 dBc!): 28.51 MHz Generated by: 2 * VCO - 4 * BFO = (2 * 24.085 - 4 * 4.915) Spurious level is affected by the TX-ALC voltage and is worst at higher TX power output. Also verified that this mixing takes place in the TX-buffer and not in the (ne601) mixer itself and is due to the broadband design of this TX-buffer (which is of course difficult to circumvent in this architecture chosen). Placing an extra set of BPF's just after the TX-mixer would solve the issue, but is not achievable in the present board layout. Of course this would not improve the SSB/audio IMD as Wayne pointed out. Because I use the K2 as driver of a 1KW+ VHF amplifier I have been very keen in getting spurious signals as low as possible. Only way I could get the spurious further down (60/70dBc) is to add an extra ERA9 amplifier (+10 dB gain) just before the transverter IF output but within the TX-ALC loop. In such a way making sure the signal level out of the TX-mixer and subsequent TX-buffer amplifier are as low as possible. Cheers, Carel,pc5m > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:elecraft- > [EMAIL PROTECTED] On Behalf Of wayne burdick > Sent: Monday, August 14, 2006 8:59 PM > To: PE1E > Cc: 'Elecraft Reflector' > Subject: [Elecraft] Re: band-passfilter location in SSB transmit chain > > PE1E wrote: > > > > > I was told that the absence of filtering between Tx mixer and Tx > > buffer > > amp could cause annoying IM products. > > ( The bandpass filters in the K2 are behind the Tx buffer amp., while > > in at > > least one of my other tranceivers in use here I see the bandpass filter > > straight behind the Tx mixer without interfacing amp. ). > > > > Could this observation contribute adversely to the SSB Tx audio > > quality ( > > in-band IM3) or do I see a ghost ? > > Peter, > > The TX buffer amp is operating class A, and any IMD it contributes will > be completely negligible compared to the IMD contributed by the > class-AB final amp. So, positioning the bandpass filters after the > buffer should have no impact on observed IMD. > > If you see any more ghosts, let me know :) > > 73, > Wayne > N6KR > > > --- > > http://www.elecraft.com > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
