Noel ) writes:
On Thu, 08 Jul 2004 11:25:32 GMT, "Frank Dresser"
wrote:

With a single IF receiver, there will be only one image. They can be either
above or below the received frequency, depending on the design of the radio.

With a single IF, what you say is true. But with a dual conversion
surely part of the design is that the image is almost inevitably out
of the coverage range of the receiver?



No, because there is nothing about double conversion that requires the
first IF to be above the signal frequency.

Early double conversion receivers would have their IF in the 2MHz or so
range. In some cases, that extra conversion would only come into play on
the highest band, 20 to 30MHz, where it was especially needed. The filtering
at that first IF was minimal, but was sufficient.

And there was a whole other design of double conversion receivers that were
common at one time. These were in effect a single conversion receiver
that covered a fixed range of 500KHz or so. The exact turning range varied
with receiver, but it was usually in the low MHz range. In order to
get other bands, a crystal controlled converter was placed ahead of this
receiver, and you'd need another crystal for each band you wanted to tune.
In some cases, the tuneable receiver covered a band that the receiver did
tune, and the converter was disabled on that band. But more commonly, it
was double conversion on each band.

When crystal filters in the HF range came along, there was a wave of ham
transceivers (and likely shortwave receivers) that had a first IF of 9MHz, but
converted to 455KHz after that, because the designer wanted the selectivity
at that lower frequency, or because they wanted to add some feature, passband
tuning or variable bandwidth selectivity, that used the extra conversion.

It's only in the past twenty to thirty or so years that upconversion to a
frequency above 30MHz became common, especially for hobby receivers.

Michael