Andrew Tweddle said:
colin wrote:
Tell us more!
Andrew
Sure,
Its quite basic, I started with this circuit but wasnt happy with it ...
http://www.elecdesign.com/Files/29/4749/Figure_01.gif
although I never bothered with the delay line, and I used a DG RF mosfet for
the high speed ramp.
I also provided a slow speed ramp generator instead of using the scope trace
output, and I used single 5v supply instaed of +-2.5v with ac coupled ip.
so I modified it, now an ECL D type flip flop with set/reset provides the
trigger wich trigers upto 2ghz or more
this holds itself in reset for 50ns then locks itself out for a further
50ns,
(it didnt like this at first with metastable states so an LC was used to
delay the feedback)
so with say a 2ghz input you get a 10mhz sampling rate,
this drives a high speed ramp wich is just an emitter follower and constant
current source down to ground,
this is tricky to get right as small ripples at the start cuase timebase
distortion
this is compared with a slow speed ramp by an ultra fast comparator,
this locks itself out so it generates a squarewave delayed from the sampling
rate signal,
the differential op from the comparator drives a 4ghz sampling unit I got
from ebay wich is just an SRD and 2 schotky diodes and 2 capacitors, this is
much better than the diode switches but it is incredibly delicate
an antiparallel schotky diode clamp is accross its input,
this just drives a fast cmos op amp with a gain of 10,
input attenuation is used to provide a maximum op of 2vp-p. and aprox 1:1
gain.
the timebase is variable but I have it at 500ps/div on my scope atm,
the slow ramp generator is set to 2ms rate, its possible to see artifacts
wich are 4ghz.
the amplitude agrees with a schotky detector upto 2ghz wich is as high as I
have used it so far.
Its made on 2side pcb made by guage isolated tracks.
It still gets modified quite often still as I think of ways of improving it,
so I havnt got a definitive circuit.
I could do with a way of checking the acuracy of the trace.
also its hard to probe around at 2ghz+, I generaly put smc coax sockets on
the pcb at various points,
and 500R from there to the point of interest.
I keep meaning to remake it with a few bells and whistles etc...
the old circuit is also along side it with the same comparator driving the
diode switches and the sampling unit.
The SRD takes about 50ns to turn off so it is delayed considerably compared
to the diode switch chanel.
im also adding a prescaler on the trigger ip for my freq counter.
Atm im using it to debug a heterodyne lidar wich uses modulated laser light
at 1ghz+ hopefully upto 2ghz.
Colin =^.^=