tip/content/eip545b_6.rst

========================================
 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:: {static}/images/eip545b/band_3_operations.png
   :alt: Simplified Band 3 Operations

For which the first step is the search for signal:

.. image:: {static}/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:: {static}/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:: {static}/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:: {static}/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).