RF performance

A key requirement when we designed the RFzero was to make spectral performance as good as possible while using the cost effective Si5351A digital clock generator. We believe we managed balancing cost and performance. However, if spectral performance is critical, e.g. for a high power beacon, transmitter or VFO then the Next Generation Beacons platform is the right choice instead of the RFzero but also in a different price range. Therefore you should see the RFzero as a low cost and fair performance RF unit.

CharacteristicsFrequencyValue (typical)Notes
Output power137 kHz
400 kHz to 200 MHz
240 MHz
280 MHz
>8 dBm
>13 dBm
>12 dBm
>11 dBm
Above 200 MHz is use above the SiLabs specifications
432 MHz using 3 x 144 MHz: > -5 dBm
1,3 GHz using 5 x 260 MHz: >- 30 dBm
No time limit unterminated on any frequency
Third order IMD
(below PEP)
1 MHz
10 MHz
50 MHz
200 MHz
35 dB/-
35 dB/75 dB
20 dB/55 dB
25 dB/-
T1: transformer/combiner
1 kHz tone spacing

For use of combiner please see the modifications page
Second harmonic137 kHz
1 MHz
10 MHz
50 MHz
200 MHz
-60 dBc
-65 dBc
-57 dBc
-42 dBc
-37 dBc
The shown frequencies are the fundamental frequencies
Please see the spectrum plots below for more details
Third harmonic137 kHz
1 MHz
10 MHz
50 MHz
200 MHz
-5 dBc
-15 dBc
-15 dBc
-10 dBc
-20 dBc
The shown frequencies are the fundamental frequencies
Please see the spectrum plots below for more details
In-band spurious1 MHz
10 MHz
50 MHz
200 MHz
-84 dBc
-75 dBc
-70 dBc
-68 dBc
+/-500 kHz from carrier
Please see the spectrum plots below for more details
Phase noise100 Hz
1 kHz
10 kHz
100 kHz
-101 dBc/Hz
-112 dBc/Hz
-117 dBc/Hz§
-117 dBc/Hz§
Measured at 25 MHz using a R&S FSIQ 26


§: Instrument limit
Ref. freq. suppression27 MHz>100 dB
Short term freq. stability*27 MHz100 mHz
10 mHz
Naked oscillator
Foam covered oscillator
Long term freq. stability27 MHz+/- 1 HzGPS better than 30 ns RMS and 99% of the time better than 60 ns

*: The RFzero has been tested generating WSPR signals on 200 MHz and receiving the fifth harmonics on 1 GHz with 100% decoding performance. On 1,3 GHz the decoding percentage was reduced to around 80%. However, this may be due to the performance of the receiver that was unlocked. The RFzero has also been tested generating PI4 signals on 108 MHz that were fed into a x96 multiplier resulting in a 100% decoded signal on 10 GHz. These tests were carried out with a foam covered oscillator.

All values have been measured with a standard RFzero, i.e. no output filter and with a T1 transformer, and using 8 mA as output drive strength except where otherwise stated.

In general it applies that if the IMD of the input signal to an amplifier is -12 dB then the contribution to the output is 6%.

Why care about spectral performance?

Way to many use the very popular Si5351A uncritically in combination with RF. The Si5351A is programmable clock generator for a digital environment where spurious are of much less concern or perhaps even irrelevant than in the radio domain. Designing RF circuits takes knowledge, time, care, resources and money.

Spectral performance, or signal purity, is expressed as

  • Phase noise
  • In-band and out-band spurious
  • Harmonics

Of these parameters only harmonics are easy to do something about with a low pass filter.

The performance is relevant both on-air but also inside a radio. When transmitting poor spectral performance means that other radio amateur stations will be disturbed more easily but also non-radio amateur receivers, radio stations, TVs, electronic devices etc. When receiving the phase noise, and spurious, both own receiver and from transmitting stations, mean that unwanted mixing takes place in the receiver resulting in unwanted signals and poorer reception of the wanted signal(s).

For more information please see e.g. Radio-Electronics.com that has a number of concise tutorials for RF.

Spectral performances

Below are a series of pictures showing both wide and narrow spectrum performances measured with a Rohde & Schwarz FSIQ 26, 20 Hz to 26 GHz spectrum analyzer with Low Phase Noise B4, FFT Bandwidth 1 Hz to 1 kHz B5, Vector signal analysis B7 and DSP & IQ extension 2 x 512 kB B70 options and with firmware options K11, K21, K72 and K74.

The measurements were carried out with a 200 MHz low pass filter mounted on the Z1-Z10 pads irrespectively on the fundamental frequencies.

137 kHz spectrum performance

137 kHz signal in a 100 kHz span.

137 kHz signal in a 500 kHz span from 0 Hz to 500 kHz.

1 MHz spectrum performance

1 MHz signal in a 100 kHz span.

1 MHz signal in a 1 MHz span.

1 MHz signal in a 5 MHz span from 500 kHz to 5,5 MHz.

10 MHz spectrum performance

10 MHz signal in a 100 kHz span.

10 MHz signal in a 1 MHz span.

10 MHz signal in a 10 MHz span.

10 MHz signal in a 50 MHz span from 5 MHz to 55 MHz.

10 MHz and 1 kHz tone spacing intermodulation performance where the T1 standard transformer has been changed to a combiner.

50 MHz spectrum performance

50 MHz signal in a 100 kHz span.

50 MHz signal in a 1 MHz span.

50 MHz signal in a 10 MHz span.

50 MHz signal in a 200 MHz span from 25 MHz to 225 MHz.

200 MHz spectrum performance

200 MHz signal in a 100 kHz span.

200 MHz signal in a 1 MHz span.

200 MHz signal in a 10 MHz span.

200 MHz signal in a 500 MHz span from 0 Hz to 500 MHz.