[GPS_Standard] Frequency problem with LTC1485 chip

[Dave Platt]

On 10/18/2014 04:16 PM, Phil wrote:
> Anyone ever have a problem with the LTC1485 (line driver) ? (U1)
> I just built the GPS Standard and I couldn't get a "lock". I started at
> the front end and I've got 10 MHZ going in and 10,000,230 (approx)
> coming out!
> I actually pulled the chip out and jumpered pin 6 to 1 (effectively
> bypassing the buffer chip) and got a brief lock.
> I'm new to the project.. any help would be appreciated!

I remember having real problems with the LTC1485 receiver when I used it
on my board - no signal at all.  The fix was simple once I figured out
the cause.

The LTC1485 and similar bus receivers are differential - they react to
the relationship between their + and - inputs.  In the VE2ZAZ design,
one of these inputs is grounded... which means that for reliable
operation, the other input must be pulled *below* ground.  Many 10 MHz
oscillators have CMOS- or TTL-compatible outputs, which do not swing
below ground.  This makes it difficult or impossible for the
differential receiver to decode the signal properly.

If the oscillator doesn't pull down below ground at all, there will be
no output from the receiver (as happened to me).  If the oscillator
pulls down to right around ground, but there's some ringing on the
signal (quite common) then you may get almost anything... including an
output signal with occasional double-pulses, resulting in too high a
frequency being detected (as might be happening in your case).

There are two possible fixes:

(1) Unground the grounded input to the LTC1485, and wire it to a
reference voltage half-way between ground and the oscillator's peak
output voltage.

(2) Don't connect your oscillator directly to the LTC1485.  Instead,
couple it through a .1 uF capacitor, and install a medium-value resistor
(4k7 or 10k or so) between the capacitor/LTC1485 junction, and ground.

The latter is easier... you could do it either on the VE2ZAZ board, or
at the oscillator, or in a wiring widget in between the two.  The
DC-blocking cap and the resistor will have the effect of shifting the
oscillator's *average* voltage down to ground - the signal will swing
between a couple of volts positive, and a couple of volts negative, at
the LTC1485 input.  This will allow the differential receiver to work
reliably.