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+========================================
+ 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).