FAQ: 24/02/2009 -------------------------------------------------------------------------- Q: Why is there an offset between some frequencies entered in the CRO and the final loced frequency? A: Due to internal pre-divider, not all frequencies can be set exactly. Only even and depending on the frequency, modulo 2,4 and 8 frequencies are possible. Therefore an offset can occure. However this offset does not influence the precision of fx. The formula is: ( (Finput + X) ) fx = real locked frequency fx = (integer) ( ------------ ) * (2 * x) Finput = CRO frequency input ( 2 * X ) X = see table below Table: --------------------------------------------------------------------- < 20MHz: X = 1 . Offset max 1Hz. Example: Finput=16.000057 MHz -> fx=16.000058 MHz Finput=10.000001 MHz -> fx=10.000002 MHz Finput=10.000000 MHz -> fx=10.000000 MHz Finput= 9.999999 MHz -> fx=10.000000 MHz Finput= 9.999998 MHz -> fx= 9.999998 MHz 20MHz < 40MHz: X = 2 . Offset max 2Hz. Example: Finput=35.123458 MHz -> fx=35.123460 MHz Finput=35.123457 MHz -> fx=35.123456 MHz Finput=25.000000 MHz -> fx=25.000000 MHz 40MHz < 80MHz: X = 4 . Offset max 4Hz. Example: Finput=70.123459 MHz -> fx=70.123456 MHz Finput=60.525000 MHz -> fx=60.525000 MHz > 80MHz: X = 8 . Offset max 8Hz. Example: Finput=108.009896 MHz -> fx=108.009904 MHz Finput=108.009895 MHz -> fx=108.009888 MHz Finput=123.444640 MHz -> fx=123.444640 MHz --------------------------------------------------------------------- Bear in mind, that the real frequency (fx) must be entered in the CRO-Online Tool. -------------------------------------------------------------------------- Q: The RF and 1PPS check says "ok", but I get only the "off-sync" message. A: Check the 1PPS signal. It must be a low pulse between 80ms and 120ms , TTL level. 100ms is perfect. A longer or shorter low pulse will cause wrong operation. ------__------//------__-----// 100ms 100ms |----1 sec------| --------------------------------------------------------------------------- Q: What is the p-factor exactly and what is it used for? A: The p-factor (Precision Factor) is an important system constant. It can be chosen by the user before he gets the characteristics of the oscillator. After that, the value should NOT be touched, except the user intends to get a new characteristics curve. In case for oscillators with lower quality (poor stability etc) the p-factor causes an adjustment of the necessary dynamic area of the phase counter to the real available dynamic area. The maximum of precision which can be achieved at all, is divided by the p-factor. That means as lower p-factor is, as higher the reachable precision. For good OCXOs like the HP10811, a p-factor of 1 can be used. For good home brewed OCXOs (like DF9LN OCXO with excellent components) a value between 5 and 10 can be achieved. Don't try to use a better p-factor as the oscillator is good for! Bear in mind: Set the p-factor, take characteristic, close the loop and forget the p-factor (except you need it for online-analysis PC-program) --------------------------------------------------------------------------- Q: How can I check if my OCXO can be used together with the CRO? A: Provided the oscillator has been wormed up (at least 1 day, if a new one is used much more longer) the CRO should be used in LOOP OPEN mode. Setup all you need and connect the oscillator to the CRO. Start the ONLINE-Analysis PC program and let it run. Observe the graph which can be seen after a while. Try to adjust the oscillator that it delivers a line as horizontal as possible. Set the scaling of the PC program as small as possible. The graph of the oscillator should not ... ... change the direction suddenly ... show jumps (occasional jumps caused by interference are ok) The oscillator should ... ... stay in direction at least for 100 sec (better longer) --------------------------------------------------------------------------- Q: How can I pull out the PLCC devices without damaging the sockets? A: Do never use an other instrument than a special instrument which is made for pull out EPLDs. Do not use a screw-driver etc. ,you will damage the socket. A broken socket will cause wrong operation of the CRO. It is almost impossible to substitute a socket!