HP 8560E 2nd IF output - Page 1
Forum section: electronics / testgear

author: kirill_ka
author_rank: Frequent Contributor
timestamp: March 13, 2019, 07:17:45 pm
content: On the first attempt when I tried to add an external IF output connector, the displayed signal dropped by ~2.5 dB. So this is caused just by reflections in the cable which I attached to the internal 2nd IF connector.It looks like the cable should be matched to the IF wavelength (310.7 MHz).Ok, I'll make another cable, but I would be grateful if someone could answer the following questions.Why didn't they isolate the external IF output?I attached partial schematic of the 2nd IF amplifier. Q803 is on the path to the 2nd IF output. C838 is connected directly to the internal 2nd IF SMB connector. Q802 is on the main signal path.What is the purpose of R823?

author: jlmoon
author_rank: Unknown
timestamp: March 14, 2019, 04:47:20 am
content: Hello,R823 in combination with C838 form a series RC network which shifts the bias on the base of Q803 depending on the frequency applied, thus changing the operating point on the load line of that transistor. Since this is a common emitter configuration, R821 & R822 set the quiescent point on the load line (bias point for Base of Q803). Depending on the frequency presented and the rules of capacitance reactance, the combination of R823 and C822 form a frequency reactive network. This RC network has frequency characteristics that attempt to keep the output linear.Hope this is a good explanationJon

author: kirill_ka
author_rank: Frequent Contributor
timestamp: March 14, 2019, 10:28:28 am
content: Thank you, Jon.Since C822 >> C821 I don't think it is designed to vary bias with the frequency. Also the frequency is fixed at 310.7 MHz centre with about 30MHz of bandwidth. However, I'm a dummy in the analogue circuit design, so I may be missing some obvious things.My guess is that R823/C822 add some negative feedback and they are there for linearity, but they don't adjust something depending on the frequency.I think, the circuit is designed to preserve the dynamic range and linearity and that comes at expense of keeping IF output not isolated from the main signal path.

author: jlmoon
author_rank: Unknown
timestamp: March 14, 2019, 02:31:46 pm
content: Yes you are correct, I wasn't sure about the IF frequencies involved. As far as the negative feedback, correct me if I am wrong, I think the combination of R824/825 provide a small amount of feedback while R823 provides a small amount of "regenerative feedback". I'm guessing to provide temperature stability for the bias point of Q803. You have me thinking now, need to drag out my textbooks and make sure of that.This is most likely the case: "This RC network has frequency characteristics that attempt to keep the output linear."Maybe some of the true eggx-spurts will chime in.

author: kirill_ka
author_rank: Frequent Contributor
timestamp: March 18, 2019, 01:03:09 pm
content: Quote from: jlmoon on March 14, 2019, 02:31:46 pmHmm. R824 and R825 are shorted with C823 and C824 for the IF frequency. And I don't understand how R823 can give a positive feedback since it's connected to the inverted IF signal.Anyway, I used 670mm* of RG316 cable to connect the external IF output. Now it introduces almost no additional loss in the main signal path if the external output is left unterminated.*Actually a little less than that to take an account the connectors.

author: jlmoon
author_rank: Unknown
timestamp: March 22, 2019, 03:55:41 am
content: Oh yes, I overlooked those two caps on the negative feed back portion of the circuit. I didn't do the calculations on those but the Xc would be quite small.Great job!