14-12
Block Diagram Theory of Operation
Block Diagram 2
Tospeed up the operation during frequency changes, a Sum Loop pretune line is provided
bythe A26 Step Loop A assembly and drives the A25 Sum Loop assembly. This pretunes
theVCO in the A25 Sum Loop assembly to allow it to lock more quickly as the A26 Step
Loop A and A27 DAC/Upconverter assemblies change frequencies.
The diagnostics procedures check the A26 Step Loop A assembly at various frequencies
but can only verify operation during static (non-hopped) operations. If the instrument
meets its specifications during static operation but fails during frequency hopping, it may
be that the A25 Sum Loop or A26 Step Loop A assemblies are slow to lock-up to the
correct new frequency. The error might appear as a high phase or frequency error at the
beginning of a new frequency hop, or the instrument may occasionally lose lock during a
frequency hop. These might be symptoms of a mis-adjusted A25 Sum Loop or possibly a
faulty A26 Step Loop A or A25 Sum Loop assembly.
A25 Sum Loop
Thisassembly contains the circuity to add together the CW signal from the A26 Step Loop
A assembly and the modulated signal from the A27 DAC/Upconverter assembly. A
pretuneline is provided from the A26 Step Loop A assembly to speed up the ability of the
A25 Sum Loop A assembly to phase lock. The output from this assembly is a 0.3 GMSK
modulated signal at 500 to 1000 MHz, depending on the frequency that was selected.
The A25 Sum Loop assembly is adjusted to match the tuning characteristics of the A26
Step Loop A assembly. Whenever either of these two assemblies are changed, it is
necessary to re-adjust the A25 Sum Loop assembly using the procedures in chapter 7.
The diagnostic program checks the A25 Sum Loop assembly at various frequencies but
can only verify operation during static (non-hopped) operations. Measurements are made
todetermine if the A25 Sum Loop assembly can phase lock and that RF power is available
on the output. If the instrument meets its specifications during static operation but fails
during frequency hopping, refer to the previous discussion about the A26 Step Loop A
assembly.
A13 Output
Themain purpose of this assembly is to provide the ability to translate the RF signal from
theA25 Sum Loop assembly to different frequency bands and to amplify the RF level. For
frequenciesbetween 500 MHz and 1000 MHz, the path through the A13 Output assembly
is “straight through” and the RF frequency remains constant. A “divide by 2” is used to
createoutput frequencies from 250 MHz to 500 MHz. For frequencies below 250 MHz, a
heterodyne section is used to mix the frequencies down to the desired output frequency.
In addition to frequency translation and level correction, the A13 Output assembly
provides AM modulation capabilities. The connections into and out of the A13 Output
assembly are shown on Block Diagram 2.
Thediagnostic procedures verify the tracking filters, ALC loop controls and, DAC values,
and measure RF output power at various frequency and power settings.