creating a 57 KHz signal from 19 KHz reference

Hi all,


I'm again experimenting with GNUradio, now with a flowgraph I found
("rds_rx") to demodulate FM/RDS.

One facts about FM/RDS:
After demodulation a FM broadcasting signal, the signal contains
multiple sub-signals, (among others) a 19 KHz 'stereo pilot' and a PSK
(RDS) signal at 57 KHz.
57 KHz is three times 19 KHz.

Question:
I am trying to find a way to use the 19 KHz stereo-pilot signal to
create a 57 KHz carrier (to down-convert the FM/RDS carrier at 57 KHz).
How do I do this?


I tried using a PLL carrier-tracking block to extract the 19 KHz carrier
(which seams to work) and direct that carrier to a 3 input multiplier
block, but that does not seams to work.
Even after filtering out DC and the carrier at  -19 KHz (negative
frequency), the output of the multiplier is always zero. (*)

Or is there a better way to generate a 57 KHz complex sine-wave from a
19 KHz reference signal?


(*)
When I just connect the output of a signal-source block (that contains a
19 KHz complex-number signal) and connect this to all ports of a 3-input
multiplier, I do get a carrier at 57 KHz.
So I am puzzled why this seams to work, but using the same technique to
multiply the output of a PLL carrier-tracking block does not result in
anything useful.


Cheerio! Kr, Bonne,