Difference between revisions of "800 MHz Exciter"

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pictures!
 
pictures!
  
The 800 exciter are similar to the other exciters with the major exception of they only have one VCO (upper) to cover the 20 MHz tuning range.  This tuning range is only 2%, where as the tuning range of a single UHF VCO is closer to 3.2% for 16 MHz of tuning range.  As is the VCO is discrete parts we have a schematic of it available.
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The 800 exciter are similar to the other exciters with the major exception of they only have one VCO (upper) to cover the 20 MHz tuning range.  This tuning range is only 2%, where as the tuning range of a single UHF VCO is closer to 3.2% for 16 MHz of tuning range.  As is the VCO is discrete parts we have a schematic of it available which really helps.
  
 
The Exciter uses the standard truth table for both the exciter ID (VHF/UHF/800/900/etc.) and the board revision.  I have little ideal what the board revision is used for.
 
The Exciter uses the standard truth table for both the exciter ID (VHF/UHF/800/900/etc.) and the board revision.  I have little ideal what the board revision is used for.
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=== Converting VCO ===
 
=== Converting VCO ===
  
IC2200 is a sealed unit containing two VCO's.  
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The main interest for us will be in the VCO section of the board.  This VCO is covered by a shield with a tuning capacitor visible through it.
  
 
Check out the schematic! The Diagram is wrong when it shows the upper and lower VCO!  
 
Check out the schematic! The Diagram is wrong when it shows the upper and lower VCO!  

Revision as of 00:53, 29 August 2016

This page covers the 800 MHz exciter in technical detail. This exciter is similar to the 900 MHz exciter and shares the VCO design. The UHF R0 exciter has this same sort of VCO design as well.

Basics

The 800 MHz exciter is pictured below. It's designed to 850-870 MHz commercial band with 12.5, 20 or 30 KHz channels using FM, VSLEP or P25 modulation formats. Like the other exciters in the Quantar series, it can switch rapidly between digital and analog formats.

pictures!

The 800 exciter are similar to the other exciters with the major exception of they only have one VCO (upper) to cover the 20 MHz tuning range. This tuning range is only 2%, where as the tuning range of a single UHF VCO is closer to 3.2% for 16 MHz of tuning range. As is the VCO is discrete parts we have a schematic of it available which really helps.

The Exciter uses the standard truth table for both the exciter ID (VHF/UHF/800/900/etc.) and the board revision. I have little ideal what the board revision is used for.

The Exciter has a CPU with firmware required to control it and reports to the station controller over a SPI bus. There is 1k of EEPROM used for storing the tuning values for the exciter and power amp in the the CPU. This allows you to tune a PA/Exciter and move it between chassis with out needing to retune. The firmware is stored on a 27C512 EPROM, and as of the current writing (Aug-2016) 020.09.018 is the most recent firmware.

The Exciter controls the PA and will shut down if the PA is not present or detects an error.


The service manual excerpt is in PDF and below.


PDF LINK

In PNG format.


Conversions

800 to 900 MHz

R1 to R2

The basic premise of converting the 800 to 900 MHz is four major parts:

  • Convert the VCO for the proper frequency coverage
  • Change the module ID
  • Change the module version ID

It's important to have a known good working 800 MHz unit before starting work. If it is not working at it's intended frequency, fix it first.

It would be good to become very familiar with the schematic and service manual before attempting this.

Tools and test equipment you should have (not an exhaustive list)

  • Hot air station
  • Under board pre-heater
  • High quality soldering iron (metcal)
  • Dual voltage variable DC supply
  • Spectrum analyzer and probes
  • Service monitor
  • Experience working with SMD and reflow techniques

Converting VCO

The main interest for us will be in the VCO section of the board. This VCO is covered by a shield with a tuning capacitor visible through it.

Check out the schematic! The Diagram is wrong when it shows the upper and lower VCO!

Reversed VCO's

The VCO substrate is ceramic, and sensitive to thermal shock, so be careful about this, as it will crack. I prefer to leave the VCO in place attached to the pins supporting it on the main PCB while working on it. To do this means the copper case must come off in one piece with out letting the board drop from the pins. Obviously the problem with this is to get the VCO hot enough to remove the case will cause the pins to reflow as well.

Prior to preheating I fashion supports out of aluminum foil or other material and jam them between the PCB and the underside of the VCO. I find one at the bottom middle and two on the top corners is sufficient. It's important there be some slight upward tension on the PCB from these supports. This will prevent the VCO board from falling off it's pins when removing the shield.

Once this is done, mask off the area using hole cut in some foil. This is a heat shield for the other parts of the board. Some times you need to use kapton tape to secure the foil in place.

UHF RX prepped for desoldering

Setup the board with an under board pre-heater set to 460f. This is important as you want the board to warm up to close to the melting point of solder. Then the hot air wand will heat it the rest of the way.

Underboard pre-heater

Once you have this preheated for a few minutes, use the hot air wand with no tip 15 L/m and 650f on the shield. Move it in a circular pattern while waiting on it to reflow. A dental pick is a good test for reflow, be aware the solder will not all flow at the same time.

Once it's flowed while keeping the heat moving on the VCO case, pull the case straight up with some needle nose pliers. The case should come clean off, though you may need to rock it back and forth a bit. Careful you don't disturb any of the VCO parts, as they will most likely be reflowed. If you do, don't panic, just look at the before picture and put everything back to how it was on the VCO. It's a pretty easy circuit to figure out.

Now the VCO should be exposed and you can tack on some test wires to it while it cools down.

UHF RX VCO test points

Hook up to the VCO as shown, we'll do the Low VCO first.

  • 8.5 V power to high and low VCO (not at the same time).
  • 0-12 V on the steering line.
  • output loops of wire on the VCO output. These are to be looped around a probe and into a spectrum analyzer to view the output.

A few points about VCO tuning:

  • The VCO will be 73.35 MHz higher than the intended receiver frequency
  • the sweet spot for the VCO steering voltage is 2.5-7.5 v
  • each VCO is designed to cover half the range of the quantar.
  • The Exciter will change VCO's when it hits the range's center frequency +25khz (UHF R2 is MHz) . You can verify this my looking at the steering voltage in the exciter as you change frequencies. A test config file (Codeplug) makes this easy.
  • with the shield on they move up about 2.5 MHz
  • from hot to cold Fr changes about 1.5 to 2 MHz. Hotter makes Fr go down, cool makes it go up.
  • removing capacitance makes Fr go up.
  • removing inductance makes Fr go up.
  • tune a bit below the frequency (2 MHz) and adjust the tuning caps
  • you may elect to move the center frequency lower on the lower VCO and higher on the high VCO if you desire a greater tuning range. I will not be showing this.
  • Get it close, I'm a perfectionist, but really 1-2 MHz from idea will not matter.
  • at higher steering voltages (>9v) the VCO may get dirty. This is normal.

Put 5.0v on the steering line and note the frequency on the spectrum analyzer. You can sweep it from low to high and check the coverage is correct per the table above (it will be about 2MHz higher with the case off).