Posted by
pc5m, Carel on
Aug 15, 2006; 8:35pm
URL: http://elecraft.85.s1.nabble.com/Up-conversion-tp392867p392878.html
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:
[hidden email] [mailto:elecraft-
>
[hidden email]] 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>
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