In message
, Tim
Shoppa writes
On Nov 29, 9:07*pm, lw1ecp wrote:
Raypsi:
- Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to
~40 is for the upstream), an impressive 16:1 range, but the L.O. is
much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O.
would sweep 1254 to 2100, 1.67:1, not enough for me.

I think maybe you're looking at the wrong generation of CATV tuners. I
think Raypsi's suggestion was for the little tin-can tuners common in
say 80's and 90's VCR's, these certainly did NOT have a 1200MHz IF,
closer to 45MHz.

In the 1980s and 90s, I was intimately involved in many types of CATV
set-top boxes from one manufacturer. I would have thought that, even
after 10 years, I would remember to the nearest Hz what the oscillator
and IF frequencies were - but I'm afraid I can't!

However, the general plan was essentially that entire input spectrum was
bandpass filtered (via a cascaded highpass and lowpass filter), and
presented to a 4-diode ring double-balanced mixer. I'm pretty certain
that one model (50 to 550MHz) had a first IF at around 650MHz, with the
LO running from 700 to 1200MHz. This was then down-converted to a second
IF - the usual 45.75MHz (for NTSC).

It was fairly obvious that you could make a wide-range variable
oscillator (say 0 to 500MHz) by 'reversing things', using a variable
oscillator (a readily achievable 650MHz to 1150MHz), and mixing it with
a fixed oscillator on 650MHz. The output of the double balanced mixer is
taken from the IF port, which (of course) goes down to DC, and is
lowpass filtered to 500MHz. As has been stated, this is essentially the
principle used by many sweep oscillators.

For most 'amateur' purposes, there is no need to have any ALC applied to
the output signal. This is achieved by ensuring that the level of the
variable oscillator is relatively high (say 66dBmV or 2V), and that of
the fixed oscillator is relatively low (say 0.3V or 50dBmV). A normal
feature of a ring mixer is that the 'signal' loss through the mixer
(typically 6dB) is more-or-less independent of level of the local
oscillator - provided it is high enough. This means that, even if the
level of the variable oscillator varies by a few dB across its tuning
range (which it is bound to do), the mixer output level stays fairly
constant 44dBmV or around 0.15V) over the entire frequency range.

But things are not perfect. Unfortunately, in the mixer, you inevitably
get mixing of the harmonics of two oscillators. The effect is to produce
unexpected signals within the wanted band 0 to 500MHz (and greatly
dependant on the actual frequency of the variable oscillator). I know of
one commercial sweep oscillator which had, at the LF end of the band,
several unwanted mixer products only 25dB down. But it was still quite
usable for most lab purposes, provided you knew of its limitations. The
cure for this inherent problem is to have the frequencies of both the
fixed and variable oscillators as high as possible (not as stated
above), but this obviously makes amateur construction more difficult.

Once you have made a satisfactory wide range oscillator, it is very easy
to convert it to a sweep generator by driving the varicap of the
variable oscillator from a sawtooth.
--
Ian