Re: AM radio receiver - design

On Feb 8, 2:18 pm, JosephKK <quiettechb...@xxxxxxxxx> wrote:
On Sat, 7 Feb 2009 10:08:22 -0800 (PST), "m...@xxxxxxxxx"

<m...@xxxxxxxxx> wrote:

<snip>

I'm pretty sure I'd hear a 3db reduction in noise with synchro. I'm
not sure why you think the snr would be 3db better. If it is because
you are listening to just one side of the AM signal, you need to
remember that you are getting half the energy as well. Thus the narrow
bandwidth doesn't buy you much unless you managed to avoid the QRM/N.
Synchro helps a bit with fading since that phenomena is due to the
mixing of signals (direct and reflected). [Narrow the BW, less room
for constructive and destructive interference.]

It's really hard to design a decent synchronous AM receiver. Most of
the built-in synch demods are real crap. Even Drake has to rev their
synch a few times. Sherwood sells an external synchro box because
everyone else does a piss poor job. Probably the R8b and AR7030 are
the only receivers out there with passable synchronous demod.

This sync-demod problem sounds counterintuitive.  Please explain why
various manufacturers have such problems.  Links to schematics would
really help.  This smacks something going on the i want to understand.

I can think of a few likely problem areas. They can be fixed but they
may give insight. There are two basic designs that I have come up
with. In one the VCO is the local oscillator and the 455KHz is a
reference making the whole thing a PLL. In the other there is a
normal AM radio up to the detector and the PLL is added at that point.

(1) In the circuits I've mess about with, the reference oscillator or
a VCO runs at twice the 455KHz number putting it at 910AM where it
would perhaps prevent the good reception of one AM station.

(2) To make the needed stability of the 455KHz, you need a trimmed LC
oscillator or a crystal. This costs a lot more than a 1N914.

(3) The servo gain crossover needs to be well below the audio but the
locking range needs to be on the order of 1KHz.


On this last point, there are a few easy fixes:

If you make the loop filter of a PLL such that it oscillates when
there is no feedback via the phase detector, it will sweep back and
forth until it locks.

When not locked, you can widen the bandwidth.

When the filtered sync demod output goes negative, you can decrease
the gain on the VCO filter.


.

Relevant Pages

  • Re: Receiver bandpass filter questions.
    ... >>I don't know much about filter design, ... basically a PLL with a logic output when the thing ... >>phase comparator output as an indication of lock. ... A PLL consists of a voltage controlled oscillator (VCO), ...
    (sci.electronics.basics)
  • Re: Crystals wanted
    ... locking a vco to the harmonics of a 3.5mhz oscillator. ... Use a divide by N circuit to divide down the vco output to ... this pll would be as clean as the crystal oscillator. ...
    (rec.radio.amateur.homebrew)
  • Re: PLL Basics
    ... PLL is a filter, any more it is a VCO. ... THEN *maybe* you can call a PLL a servo and a filter acting ...
    (comp.dsp)
  • Re: PLL Basics
    ... PLL is a filter, any more it is a VCO. ... a PLL produces an output signal when the input is zero. ...
    (comp.dsp)
  • Re: PLL Basics
    ... PLL is a filter, any more it is a VCO. ...
    (comp.dsp)