changeset
[eip545b] part 6
Tue, 28 Aug 2018 15:12:49 +0200
- author
- David Douard <david.douard@logilab.fr>
- date
- Tue, 28 Aug 2018 15:12:49 +0200
- changeset 110
- 64b852fb7470
- parent 109
- 96e9f64b86ed
- child 111
- 25cae2c53694
[eip545b] part 6
| content/eip545b_6.rst | file | annotate | diff | comparison | revisions |
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/eip545b_6.rst Tue Aug 28 15:12:49 2018 +0200 @@ -0,0 +1,112 @@ +======================================== + EIP 545B RF Frequency Counter - Part 6 +======================================== + +:author: David Douard +:Category: Electronics +:Tags: test equipment, RF, EIP, 545, 545A, 545B, 575A, 578A, counter +:series: EIP545B Frequency Counter +:series_index: 6 + + +Band 3 Sensitivity +================== + +I have been trying to figure out a bit more about the `sensitivity problem +<{filename}/eip545b_3.rst#sensitivity-problem>`_ described in `part 3 +<{filename}eip545b_3.rst>`_. + +The main problem remains the first YIG sphere that has fall off the holding +rod. I have tried to manipulate it with thin wood stick, under my cheap USB +microscope, but as expected, this operation is *very* difficult. + +So for now, this is not fixed at all. + +However, I was wondering if this YIG sphere was the only problem involved in +this lack of sensitiy. + +As far as I can understand how the unit works, there are 2 level-related output +signals from the A203 assembly: the Band 3 RF level signal and the IF threshold +signal. + +The overall process of the Band 3 is as follow: + +.. image:: {filename}images/eip545b/band_3_operations.png + :alt: Simplified Band 3 Operations + +For which the first step is the search for signal: + +.. image:: {filename}images/eip545b/band_3_search.png + :alt: Band 3 Search for Signal + +I seems to me that the RF level is used for two purposes: in the search for +signal operation first, to measure the RF level then. + +The RF level is measured from the so called "Video Amplifier", which can has a +gain of around one, or can be set to +15dB. + +So another possible culprit could be the this section of the Video Amplifier. + +Unfortunately, the schematics of the A203 assembly is not available in the +service manual: + + The assembly drawing and schematic for both the VCO and IF circuits are not + available. The entire A203 assembly must be tested as a complete unit to + ensure proper performance of the counter. Repair of the Microwave (YIG) + module can only be done at the factory. The VCO and IF Amplifier boards + require special test equipment, therefore field repair is not recommended. + +RF and video amplifier +====================== + +In order to understand, I have drawn the schematics of the board on which are +the IF amplifier and the video amplifier: + +.. image:: {filename}images/eip545b/A201_2020303.jpg + :alt: A201 RF amplifier board + +Fortunately, the board is quite simple, and despite being made of SMD +components, single sided. + +The schematics looks like: + +.. image:: {filename}images/eip545b/A201.svg + :alt: Schematic of the A201 RF amplifier board + +The video amplifier consist of a simple MC1458 opamp (mainly a dual 741). + +The 15dB gain is selected by an input pin, leveled by the first half of the +MC1458 (used as a comparator) driving of transistor. The marking on the SOT-3 +package is 6A Y, which is not easy to find. I finally figured it might be +2SJ106, a JFET transistor. + +Before finding the kind of transistor it is, I though it was faulty, because a +simple measurement of the DS with an ohmmeter gives a low value (around 200 +ohms). It took me a little while to rememeber it's a normal behaviour for this +kind of transistor (the Drain and the Source being two sides of a single piece +of silicon). + +I removed it from the board, but a quick check using a 20$ LCR/transistor meter +showed it looks fine. + +However, I also need to have a rough understanding of the YIG filter and mixer +assembly. + + +YIG filter filter and mixer assembly +==================================== + +The YIG tuned filter and mixer assembly looks like: + +.. image:: {filename}images/eip545b/YIG-mixer.jpg + :alt: YIG filter output and mixer assembly + +Once again, I am trying to understand what this whole thing works, but I really +don't have the knowledge to do so, really. + +Looking at this macro photo of the output stage of the YIG filter, it looks +like a single diode balanced mixer. The LO signal, provided by the VCO, is +coupled with the input signal comig from the YIG filter. + +It looks to me that the first square block after the coupler could be a mixer +diode (otherwise I really don't understand how this could act as a mixer).
