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Testing Session
Next test session is Sept 8th


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Agile Modulator Expirements


I recently got one of these from a friend who bought it on e-Bay. These devices are complete vestigal sideband video modulators in a rack mount chassis. The 'agile' term refers to the fact that the RF output frequency of the device can be set via front panel switches. This devices are designed to be used in a CATV head end installation, and as such, output low level RF, on the order of ~+10dBm (~60dBmV)

Users manuals for these things are practically non-existant, as the company who manufactured them has been out of business for many years now. Google searches turn up little info, so I just hooked it up and started playing. Here are my results.

Front Panel Info:

Front Panel Indicators and Controls: L-R
POWERPower On/OffLocking Toggle
MOD ADJ, VIDEOAdjustment of video input levelTrimpot
MOD ADJ, AUDIOAdjustment of audio input levelTrimpot
OVER MOD, VIDEOIndicates when sufficient video is presentLED
OVER MOD, AUDIOIndicates when sufficient audio is presentLED
 Shows the RF TV Channel NumberLED Display
ChannelSelects the output RF TV Channel frequencyDipswitches
RF LevelAdjustment of RF Output levelTrimpot

Rear Panel Info:

Rear Panel Indicators and Controls: L-R
OUTPUT, MONITORMonitor port, 20 db down from main output'F' Connector
OUTPUT, RFMain RF output port'F' Connector
IF OUT, VISUALIF Output port for visual component, jumpered to IF IN, Visual'F' Connector
IF OUT, AURALIF Output port for aural component, jumpered to IF IN, Aural'F' Connector
IF IN, VISUALIF Input port for visual component, jumpered to IF OUT, Visual'F' Connector
IF IN, AURALIF Input port for aural component, jumpered to IF OUT, Aural'F' Connector
AUDIO INInput for audio baseband signalTerminal Board
AUDIO INInput impedance select for audio baseband signalSWITCH
VIDEO INInput for video baseband signal'F' Connector
FUSEProtection deviceFUSE
POWER CORDFor connection to AC mainsPower Cord
AC OUTLETA source of 120 VAC to power auxillary equipmentOUTLET

Initial Setup:

Connect your stations audio signal (line level input, no mikes) to the audio input terminal board. Select the appropriate input impedance of the driving device, e.g. line output from mixers and cameras should probably select 10K. Connect your baseband NTSC signal from camera or SEG to the Video input jack. If your station uses RCA or BNC's you'll have to adapt this to an 'F' connector. Connect AC plug to a source of 120 VAC and power on the unit. You should see the channel display lite up.

With your normal sources of video and audio applied (use a tone for audio, if needed), adjust the video modulation adjustment trimpot on the front panel up until the led just comes on and then back off a little. Do the same for the audio signal.

Tuning Information:

To tune the GL2610XT, adjust the dipswitches on the front panel to correspond the desired output channel. The dipswitches are arranged in a pattern of two rows, one group of eight to the left, and a group of ten to the right. On my unit the first group of switches to the left is labeled (L-R) 1 through 8, and these correspond to the ten's digit of the channel number. The second set of dipswitches to the left is labeled (L-R) 0 through 9 and correspond to the unit's digit of the channel number. To select a particular channel just set the switches for the desired channel's digits to the up position. All other switches must be in the down position or you'll get an error (the channel indicator shows 'E'). For example, to program Channel 34, switch 3 in the first (left) group and switch 4 in the second group (right) would be in the up position. See the chart below for a few of the popular output channels.

ChannelCarrier FrequencyDipswitch Setting
57421.25000001000 0000000100
58427.25000001000 0000000010
59433.25000001000 0000000001
60439.25000000100 1000000000

Note: Setting my unit for Channel 60 causes an error on the display for some reason.

Output hookup:

The RF output is available at the RF OUT 'F' jack on the rear panel of the modulator. You can hook this up to a simple antenna inside the house for testing purposes. The RF output level can be adjusted via the front panel RF LEVEL control. Realize that this is a low level signal, around 10 to 20 mW. You can see this across the room quite easily with a regular ATV downconvertor box, but I suspect that the range would be extremely limited once the output is run to an external antenna. Tom O'Hara, W6ORG, of PC Electronics, has an app note available that talks briefly about using the output of an agile modulator to drive his external PA5 20W PA brick ("Driving the PA5 20 Watt amp from 60 dBmv cable TV modulators"). It looks like that even with the output turned up to max on the GL2610XT, it will only drive the external power amp to about 10W. I am thinking that a simple power gain stage of about 10db can be added to drive this to full output. I am prototyping one using a Mini-Circuits MIMIC, see the ASCII drawing below. This is very preliminary and it's quite possible the device I selected will not work in this application. I have'nt gotten around to doing it yet, so more on that later!

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