Jim,
Good points, although as I look at the HEROS passpand plot -- and although
it may first appear wide on the graph, it looks reasonably close to the
preselector response we see from Collins 75S or Drake R4 receivers. Recall
that in these receivers, a bank of moving iron core slugs establishes the
filter peak through mechanical rotation of a "preselector" control. The
HEROS unit does the same through a "9-bit digital variable capacitor." I
would expect the peak to be even more compressed as it's tuned lower in
frequency like on the 80m and 160m bands.
One benefit of the HEROS unit over a large, mechanical filter bank is that
it allows for tracking as the receiver's frequency is changed. The HEREOS
unit will connect to several SDRs via DDUtil. So, DDUtil pings the receiver
and tells the HEROS unit to tune at particular frequency. Apart from
changes in response seen on a spectral display, the user hears no change as
the filters automatically track.
I should have mine in a week or two and I'll compare against the two old
receivers mentioned. Although I didn't purchase mine to minimize
cross-band interference, my hope is for >10 dB of difference between the CW
and BCB portions of the 40m band. Is 10 dB enough in Europe on 40m? That
depends on the basic performance of the receiver. Maybe Rick and Steve
(G3TXQ) can chime in with their input.
Paul, W9AC
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Friday, September 16, 2016 5:17 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Preselector for SDR
On Fri,9/16/2016 9:28 AM, Rich Jones via TenTec wrote:
> QEX magazine May/June 2008 M0MWA presents a computer controlled
> turnable bandpass filter (preselector)
There is an important difference between a pre-selector and a bandpass
filter, at least for the application that Rick has raised -- it's bandwidth.
The issue that he has raised, and it can be a VERY big deal in some parts of
the world, is that there are high power broadcast stations either within or
quite close to some ham bands. This condition exists in EU and AS on 40M,
and in various parts of the world on 20M.
Bandpass filters are, in general, designed to have very low SWR throughout
the band of interest, and to reject the adjacent ham bands.
Their alignment is fixed (adjusted on the bench with a swept analyzer,
preferably a VNWA so that it also displays return loss). Their skirts are
rarely very steep, and I've seen none that are steep enough to reject those
strong BC stations adjacent to 20M. The BC stations on 40M are INSIDE the
passband of 40M bandpass filters.
To work in these conditions, the filter must have very high Q, and must be
tunable, and there must be sections for each band that can be switched as
needed. THIS is what we generally call a pre-selector.
73, Jim K9YC
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