volume55-number1 - Flipbook - Page 6
ADC Full Scale (dBFS)
Back-Off (dB)
Always Multisignal
Interferers Present
Pk-Ave Head Room (dB)
Max Operating (dBFS)
Amplitude
SFDR (dB)
Min Detectable
Signal (dBm)
6AB'CD+&ED,5*'.*/0
S/N
Threshold (dB)
Receiver Sensitivity (dBFS)
Max IMD (dBm)
Receiver Noise Floor (dBm)
Noise Bandwidth (dB)
ADC NSD (dBFS/Hz)
ADC Device Noise (dB)
Thermal Noise (dBFS/Hz)
Frequency
Figure 1. Relating SFDR to ADC operating range, noise, IMD spurs, and detection threshold.
F3 + F2 – F1
F3 + F1 – F2
2F1
F1 + F2
2F2
F3 – F1
F3 – F2
2F2 – F1
iSFDR
PN
F3
F2
2F1 – F2
Magnitude (dBm)
F1
Output Spectrum
0
1k
2k
3k
4k
5k
6k
7k
8k
Frequency (MHz)
Figure 2. An example of multisignal F1, F2, and F3 (60 MHz each) inducing second-harmonic, IMD2 (red), IMD3 (green), and IMD2/3 combo (gray) spurs. The noise floor (brown) is noted as PN.
Multi-Octave IMD2 Challenges in Wideband
Digital Receivers
Broadened SFDR Definition for Wideband
Digital Receivers
The wideband digital receiver evolution introduces new RF challenges. Multisignal
second-order intermodulation distortion (IMD2) spurs emerge as problematic
dynamic range impairments in the multi-octave wideband digital receiver. While
IIP3 has long been a key figure of merit (FOM) in RF device data sheets, IIP2 is
harder to track down and can be more problematic to the EW designer. The
problem with IMD2 spurs is that they only fall off by 1 dBc for every 1 dB decrease
in the incident 2-tone signal power, while third-order intermodulation distortion
(IMD3) spurs fall off by 2 dBc.
IMD2 crashing the party requires a refreshed definition of the popular receiver
FOM instantaneous spurious-free dynamic range (SFDR). SFDR specifies how far
down a receiver can detect a small signal when there are multiple larger signals
creating IMD spurs. SFDR is specified in dB relative to the large signals.
Of course, multi-octave direct RF sampling at the lower portion of the ADC first
Nyquist zone is nothing new. For example, an older system might sample at
500 MSPS and observe dc to 200 MHz in the first Nyquist zone with no IMD2
problems. This is because at these lower frequencies (that is, less than a few
hundred MSPS), ADC characteristics are highly linear and the effective IIP2 and
IIP3 of the ADC is very high, resulting in benign IMD2 products invisible below the
noise floor. Just like in wideband RF devices, however, multi-GHz, multi-octave
ADC linearity will degrade with increasing frequency, and IMD2 products will
often sit above the noise floor at higher operating frequencies. Going forward,
we’ll need to deal with IMD2.
6
Traditionally, SFDR is defined in terms of IMD3 products, along with NF and
processing bandwidth. IMD3-referenced SFDR is derived in many texts, and is
sometimes clarified as instantaneous SFDR, which is what we mean in this
article.5,6 We’ll call it SFDR3:
SFDR3 dB = 2/3 [IIP3 dBm – PN dBm] – [S/N threshold dB]
PN = –174 dBm/Hz + NF dB + 10Log10[Bv/Hz] dB
(1)
Bv = processing bandwidth Hz
Today IMD2-referenced SFDR receives less attention, but it is looming on the
horizon as a major impairment needing mitigation. It can be derived in the same
manner as SFDR3. Here we’ll call it SFDR2:
SFDR2 dB = 1/2 [IIP2 dBm – PN dBm] – [S/N threshold dB] (2)
Figure 2 illustrates an RF front-end spectral scenario whereby three simultaneous signals (F1, F2, and F3) create intermodulation products that set the lower
bound to dynamic range. Below this level, the wideband digital receiver can’t
easily tell whether a target is real or a false IMD spur.
Analog Dialogue Volume 55, Number 1
