Instrument Function > Modulation > Frequency Modulation (FM) > Frequency Modulation Settings
Frequency Modulation Settings

To access the "Frequency Modulation" dialog, select "Modulation > Configure > Frequency Modulation" or use the MENU key under "Modulation".

In the upper section of the dialog, you can select the modulation source and activate modulation. The modulation source can be selected independently for the different modulation types and the LF output.

For a fully equipped instrument two LF generators and a noise generator are available as internal sources.

State

Activates frequency modulation.

Activation of FM deactivates phase modulation.

Remote command: 
 
FM Source

Selects the source for the FM signal.

You can select the source independently from the different modulation types and the LF output. The configuration of the selected external and/or internal modulation source is performed in the lower section of the dialog or in the "Mod Gen block" (internal source only).

A fully equipped instrument provides two LF generators as internal modulation signal sources.

"Internal"

Uses the internal LF generator as modulation signal source for FM.

"External"

Uses an externally applied modulation signal.

The external signal is input via the FM/PM EXT connector.

"Internal + External"

Uses both, the internal and externally applied modulation signal, for example to perform two-tone FM.

"Ext digital"

uses an externally applied digital modulation signal. The signal is input via the AUX I/O connector.

This mode is provided for fast frequency hopping. The FM-Data bus is directly connected to the DDS of the synthesizer module. Thus, very fast frequency hopping (settling times of <=10 us) is possible for frequency bandwidth up to 40/80 MHz, depending on the RF frequency. The accuracy of the set frequency deviation is limited to 50ppm.

The binary format of the external digital data can be selected (see "Binary Format").

Remote command: 
 
FM Mode

Selects the mode for the frequency modulation.

"Normal"

The maximum range for modulation bandwidth and FM deviation is available.

"Low Noise"

(requires option R&S SMA-B22)

Frequency modulation with phase noise and spurious characteristics close to CW mode. The ranges of modulation bandwidth and FM deviation are reduced (see data sheet).

If your instrument is equipped with the SynEx Assembly (part number 1413.1800.02), you can reduce the phase noise even more by setting the PLL bandwidth to "Narrow". See "Main Pll Bandwidth" and "The Configurable Main PLL Bandwidth" for details.

Remote command: 
 
FM Deviation

Sets the modulation deviation in Hz.

The deviation is given as the absolute deviation in relation to the set center frequency, for example a deviation 5 MHz at a center frequency of 1 GHz leads to a frequency modulation between 995 MHz and 1005 MHz.

The maximum deviation depends on the RF frequency set and the selected modulation mode (see data sheet). It is possible to enter a deviation that is too high for a certain RF frequency or to vary the RF frequency to a range in which the deviation can no longer be set. In this case the maximum possible deviation is set and an error message is displayed.

The deviation of the internal source must not exceed the deviation of the external source in case of modulation source "Int+Ext".

Remote command: 
 
Internal Source

Selects the internal signal source to be used for frequency modulation. The available LF generators provided in the list depend on the installed options.

"None"

Deactivates all internal modulation sources.

"LFGen1 / LFGen2"

Uses a signal generated by one of the internal LF generators as modulation signal.

"LFGen1+2"

Uses the signals of both internal LF generators for frequency modulation.

You can set the LF frequency and modulation deviation separately for each generator. The sum of the deviations of the two modulation generators must not exceed the total value set with "FM Deviation".

With this setting, you can perform two-tone FM modulation.

"Noise"

Uses the noise signal for phase modulation, that is white noise either with Gaussian distribution or equal distribution.

This setting affects all analog modulations which use the noise generator as the internal modulation source.

See also "Noise Settings".

"LFGen1+Noise / LFGen2+Noise"

Uses a signal generated by an internal LF generator and overlaid with white noise for phase modulation.

Remote command: 
 
Deviation 1/Deviation 2

Source Internal, LFGen1+2 only

Selects the deviation for the two frequency modulation signals. The sum of the two values must not exceed the overall modulation deviation set in the upper part of the menu.

Remote command: 
 
LF Gen 1/2 Freq

Source Internal, LFGen1+2 only

Sets the frequency of the two LF generators.

Remote command: 
 
LF Gen 2 Shape

Source Internal, LFGen2 only

Selects the shape of the second LF generator signal. In case of selection Trapeze, additional settings concerning the duration of the single trapeze elements can be made (see "LF Output Dialog").

Remote command: 
 
FM Sensitivity

Displays the input sensitivity of the externally applied modulation signal at the FM/PM EXT input in Hz/V.

The modulation deviation entered with "FM Deviation" is achieved with 1 Volt (= Upeak) of the input signal.

Note: The input voltage should not exceed 1.1 Vp otherwise modulation distortions might occur.

Remote command: 
 
Adjust FM Offset

Starts the adjustment for the FM/PhiM modulator. The option is adjusted with respect to DC-offset.

Remote command: 
 
FM External Coupling

(Source External only)

Selects the coupling mode (AC or DC) for the externally applied frequence modulation signal.

Note: Coupling for external feed via input FM/PM EXT can be set independently for all modulations using the external modulation signal.

"AC"

Disconnects the DC voltage component and uses only the AC component of the modulation signal.

"DC"

Uses the modulation signal with both components, AC and DC.

Remote command: 
 
Ext. Impedance

(Source External only)

Sets the impedance for an externally applied modulation signal. Use the AM EXT connector for amplitude modulation signals, and the FM/PM EXT frequency of phase modulation signals.

You can select 50 Ohm or high (>100 kOhm).

This setting affects all analog modulations which use the external modulation signal.

Remote command: 
 
Binary Format

Source External only

Selects the binary format for external digital frequency modulation (see Table "Binary data formats"). The data is input via the AUX I/O interface.

NOTICE! Risk of instrument damage!. The maximum possible input voltage is 3.3 V DC.

Binary data formats

Scale

Example for set frequency deviation = 10 MHz

Offset Binary

Two's Complement

+ full scale

10 MHz

1111….1111

0111….1111

+0.75 full scale

7.5 MHz

1110….0000

0110….0000

+0.5 full scale

5 MHz

1100….0000

0100….0000

+0.25 full scale

2.5 MHz

1010….0000

0010….0000

0

0 MHz

1000….0000

0000….0000

-0.25 full scale

-2.5 MHz

0110….0000

1110….0000

-0.5 full scale

- 5 MHz

0100….0000

1100….0000

- 0.75full scale

-7.5 MHz

0010….0000

1010….0000

- full scale + 1LSB

- 9.999695 MHz

0000….0001

1000….0001

- full scale

- 10 MHz

0000….0000

1000….0000

Pin connection for input of digital FM data via AUX I/O connector

Connection

Parallel data input with 16-bit word width for digital FM modulation data.

11 - FM_DAT

FM_DATA 15 (MSB)

12 - FM_DAT

FM_DATA 14

13 - FM_DAT

FM_DATA 13

14 - FM_DAT

FM_DATA 12

15 - FM_DAT

FM_DATA 11

16 - FM_DAT

FM_DATA 10

17 - FM_DAT

FM_DATA 9

18 - FM_DAT

FM_DATA 8

19 - FM_DAT

FM_DATA 7

20 - FM_DAT

FM_DATA 6

21 - FM_DAT

FM_DATA 5

22 - FM_DAT

FM_DATA 4

23 - FM_DAT

FM_DATA 3

24 - FM_DAT

FM_DATA 2

25 - FM_DAT

FM_DATA 1

26 - FM_DAT

FM_DATA 0 (LSB)

"Offset Binary"

Offset Binary code is selected. This code represents analog values between full scale and minus full scale.

"Two's Complement"

Two's Complement code is selected.

Remote command: 
 

 


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