Setting up the PNA LO for an 8.33 MHz IF signal
The PNA LO must be set so that an 8.33 MHz IF signal is produced by the mixers for
input to the PNA Option H11 inputs. Using the equations below, the appropriate LO
frequency can be calculated. (To set up the PNA-X LO for an IF signal, in the procedure
below, simply change “8.33 MHz” to “7.605634 MHz “.)
Using the rear panel LO available with Option H11 as the LO input of
the 85309B:
We know that for a mixer, IF = N(LO) –RF
where N = external mixer harmonic number
With Option 080 (frequency offset) the frequency out of the rear panel LO port is defined
as, LO = m
d (RF) + 8.33 MHz
Substituting for LO in the first equation we have,
IF = N ( m
d (RF) + 8.33 MHz) – RF
= ( m
d
)N(RF) + N(offset) + N(8.33) – RF
To create a low side LO, set m = 1 and d = N
Simplifying, IF = RF + N (8.33) + N(offset) – RF
= N(8.33) + N(offset)
Since IF must be equal to 8.33 MHz, then:
8.33 = N(8.33) + N(offset)
(1 – N)8.33 = N(offset)
Therefore, offset (MHz) = (
1 – N
N
) 8.33
Using the Option 080 dialog box (shown in Figure 25) to set up the LO, enter the offset
calculated above, set Multiplier to 1 and Divisor to N (the harmonic number of the exter-
nal mixer) and select the box next to Frequency Offset on/off, then click OK.
Figure 25. Option 080 dialog box.
Note: The following equations are not
required for frequencies under 20 GHz. At
lower frequencies, the PNA operates in fun-
damental mixing mode and the LO frequency
is automatically offset by 8.33 MHz.
41 | Keysight | Antenna Test – Selection Guide