Gauge Wideband LO Noise In Passive Tx/Rx Mixers

Passive integrated-circuit (IC) mixers are widely used in cellular base-station transceivers because of their low noise and outstanding linearity. Suitable for both downconversion in receivers and upconversion in transmitters, these mixers nonetheless can be hindered by wideband noise boosted by buffer amplifiers used to increase local-oscillator (LO) levels. Fortunately, a single noise parameter for passive mixer ICs makes it possible to calculate system-related impairments when using these ICs in base-station transmit and receive applications.

Ideally, a cellular base-station transmitter should restrict all energy within the required frequency allocation. Unfortunately, even without spectral regrowth from amplifiers, wideband residual phase noise present in upconverted transmit signals can cause problems for co-located receivers at a given site. Although at a lower level than the transmitter oscillator's close-in phase noise, this broadband noise still may be at a sufficiently high level to desensitize a co-located receiver.

In conventional discrete passive diode or FET mixer cores used in base-station transmitters, LO ports are matched to 50 Ω and filters can be used to reduce noise prior to applying a signal to the LO port. But in integrated mixers with internal LO driver stages, thee is not the same luxury of 50-Ω access. Signal quality can be degraded by this internal circuitry, with upconverted signals taking on the spectral skirt and noise floor of the LO buffer amplifiers. Specifiying and designing for lower wideband noise in integrated LO buffers produces lower out-of-band transmit noise. This can ease the rejection requirements of high-Q transmit filters and diplexer filters in front-end equipment. But still, it is useful to know the effects of this broadband noise on downconverter and upconverter performance.

Cellular base-station receivers must deal with high-level blocking interferers when receiving weak in-band signals. The blocking signals reciprocally mix with noise in the LO at the mixer core and increase the noise floor within the frequency band of the intermediate-frequency (IF) output. Fortunately, a single parameter can be deduced to describe the effects of this wideband noise for passive IC mixers as downcoverters in receivers and as upconverters in transmitters.

Passive diode and FET ring mixers are commonly used in cellular base-station receivers (Fig. 1). They require large external LO drive levels (typically +17 dBm) to achieve high third-order-intercept (IP3) performance. These mixers work with discrete IF amplifiers driving surface-acoustic-wave (SAW) filters and require discrete LO buffer amplifiers to achieve acceptable LO drive levels. Although active IC Gilbert-cell mixers can provide reasonable levels of gain, they lack the linearity requirements of modern cellular base stations.^2,3

However, recent advances in silicon mixer ICs have resulted in devices capable of the linearity (IP3 of +34 dBm) and low-noise (noise figure of 7 dB) performance levels required by demanding base-station applications. These mixers have internal LO drivers, obviating the need for large-signal external driver amplifiers. The passive mixer ICs are reciprocal devices unlike Gilbert-cell mixers, allowing them to work as both upconverters and downconverters. With cascaded IF amplifiers, they can yield high IP3 performance (+26 dBm) and low noise figures (less than 10 dB) and have sufficient gain to offset SAW filter loss in receiver front ends. Figure 2 shows the functional block diagram of a typical high-dynamic-range mixer IC. These devices can work with LO levels as low as –3 dBm. They are available in small-footprint (5 × 5-mm) QFN packages with form factors smaller than their discrete counterparts.

Thermal noise is the most commonly specified and measured type of noise in downconversion (receiver) mixers. It describes the noise performance of a mixer that has a 50-Ω matched RF input port with a noise power density of –174 dBm/Hz (kT₀). Input-referenced thermal noise can be extracted from a mixer's noise figure (10log₁₀ F) specification by:

where:

k = Boltman's constant (1.381 × 10^–23 J/K) and

F = the noise factor.

The noise factor (F) of a passive mixer is

where:

L_c = mixer conversion loss at a given temperature, T_p(K).

In the receiver path, reciprocal mixing occurs in the presence of strong RF signals at a mixer's RF port—noise that is not accounted for during noise-figure measurements. Input-referenced reciprocally mixed noise, Nⁱ_rm, can be evaluated at a specific blocker level, S_bl. Given an LO noise floor of L into the mixer and a bandwidth of B, the reciprocally mixed noise at IF is

The phase noise is assumed flat if the interferer frequency offset is a sufficiently large offset from the desired signal. These two noise sources are independent⁴ and can be summed as shown in Fig. 3. The signal-to-noise-ratio (SNR) degradation from the input to the output in the presence of blockers can be expressed as

Substituting Eq. 3 into Eq. 4 gives an expression for the effective SNR degradation taking thermal and LO noise into account:

Receivers are primarily specified for sensitivity and allowable impairments to reception due to their nonideal behavior. For example, in a GSM system (Fig. 4), the base station should be able to receive a signal at a level of –104 dBm with a specified maximum allowable error rate. In the presence of interfering tones, GSM base-station receiver sensitivity should degrade by only 3 dB. These blocker tone levels (S_bl), along with the desired sensitivity (S_des), modulation bandwidth (B), and required carrier-to-interference ratio at the mixer output (C/I), dictate the noise floor (L) specifications.⁵ For the GSM example, S_bl = +13 dBm, S_des = –101 dBm, and B = 200 kHz; with an allowable C/I ratio of 10, L is –151 dBc/Hz.

Base-station transmitters must operate with signals that comply with a spectral mask for in-band and out-of-band signals. GSM specifications call out –98 dBm as the maximum allowed transmit energy in the receive band.⁸ If the base station transmits, for example, +43 dBm (20 W) power with wideband noise of –160 dBc/Hz, then –117 dBm/Hz (43 – 160) energy spills into the bandwidth of the co-located receiver. The integrated noise level into a GSM receive band (B) or 200 kHz is –64 dBm. This noise represents unwanted interference in the receive band and is 50 dB above the minimum receivable signal level of –140 dBm. Diplexers that connect both the transmitter and the receiver to a single antenna must provide sufficient blocking of transmit noise from –64 dBm to well below –98 dBm. As more wideband noise is generated in the transmit mixer IC, more filtering in receive band will be required of the diplexer.

LO buffer amplifiers in high-linearity passive mixer ICs are designed to provide constant high-level drive into the mixer cores, over a range of input signal levels. The outputs of these buffers are high-level signals that directly drive the mixer cores in order to achieve high linearity. The saturated LO buffers used in passive mixer ICs degrade the wideband SNR of the filtered low-level input. The wideband noise floor can be filtered to –174 dBm/Hz. With a signal level of 0 dBm, the wideband SNR is –174 dBc at the input of the IC's LO port. Practical IC LO large-signal buffer amplifiers must not degrade this ratio to below –155 dBc/Hz in order to meet system requirements. These buffer amplifiers are part of the IC's internal circuitry, as part of a non-50-Ω monolithic circuit, and a user does not have access to the LO buffer outputs (which directly feed the mixer circuitry). Still, it is possible to measure the SNR degradation of these buffer amplifiers. This degradation in a receiver mixer has been characterized by using a blocked signal and measuring the noise output at the 50-Ω IF port. The characteristic parameter, L (in dBc/Hz), described in Eq. 4 can be deduced from this noise measurement:⁴

Figure 5 shows the RF-to-IF SNR degradation of a PCS/DCS/UMTS-band passive mixer-based downconverter, the model MAX9994 from Maxim Integrated Products (Sunnyvale, CA), as a function of the blocking level. This is a representation of Eq. 4 as a function of LO noise L (in dBc/Hz). Four different noise regions are identified in the figure. At low RF blocker levels, the SNR degradation is mostly due to thermal noise (F). Thermal noise is the noise figure that is commonly referred for mixers. As the RF blocker level increases, the mixer enters into region 2, where thermal noise and reciprocally mixed LO noise contribute equally to the SNR degradation. Region 3 is a straight-line portion of the characteristic where the SNR degradation is mostly determined by LO noise. A base-station receive mixer is designed to handle RF blocker levels in region 3. The data points indicate good match between simulation and measurement results versus the model described by Eqs. 3 and 4. In region 4, a deviation between measurement data and the characteristic curve is apparent, which is due to compression effects not accounted for in the model.

Passive Mixer
The MAX 9994 IC is a passive mixer in cascade with an IF amplifier.⁷ The downconverter is designed for nominal gain of 8.5 dB, noise figure of 9.5 dB, output power at 1-dB compression of +13 dBm, and current draw of 220 mA. The input third-order intercept point (IP3) is nominally +26.5 dBm. The SNR degradation under RF blocking conditions can be measured using the setup described in ref. 4. The SNR_in/SNR_out with an RF blocker level of +5 dBm is 19 dB. This was noted by measuring the noise floor of the downconverted signal under RF blocking conditions. This point lies on the L = –160 dBc/Hz curve of Fig. 5. This region is ideal for characterizing LO noise (L) since the buffer amplifier noise is the dominant contributor for the cumulative SNR degradation and thermal noise can be ignored as a first-order approximation. The LO noise can be cross-checked from the SNR degradation of 19 dB. Referring this noise to the input results in N_i = –174 + 19 = –155 dBm/Hz. Since the blocker level used in this analysis was +5 dBm (Si), the value of L is –160 dBc/Hz.

Another IC, model MAX2039, employs a passive FET mixer with the same LO buffer amplifier as the MAX9994. It can be used as an upconverter or a downconverter, and exhibits the same conversion loss (L_c) of 7 dB in either case. As a downconverter, the IP3 is +34.5 dBm while as an upconverter the IP3 is +33.5 dBm. When used as an upconverter, the same LO noise parameter determined by receiver measurements in the above receiver case can be used to determine the wideband output noise floor at the RF port. For this to be true, the reciprocal mixing of the LO buffer amplifier noise with the input RF blocker in the downconverter should be the same as the reciprocal mixing of the IF signal with the noise that ends up at the RF transmit port. If L can be measured in the MAX9994, which uses the same passive mixer and buffer amplifier as the MAX2039, then it should be possible to use the same value of L to deduce the wideband transmit noise of the MAX2039. The objective here is to use L determined by the receiver measurement to deduce the transmit noise and verify the transmit noise calculations by means of measurements.

In the presence of a blocker in region 3 of the characteristic curve, for example at an RF power level of +5 dBm, the IF amplifier is not compressed. The noise floor of the output of the passive mixer in the MAX9994 is high (P_in – L_c + L = 5 – 7 + 160 = –158 dBm/Hz) compared to the input referred noise of the IF amplifier (2.5 – 174 dBm/Hz. This noise simply gets amplified by the IF amplifier and ends up at the output of the MAX9994. Thus, the LO noise measurement of the passive mixer portion of the MAX9994 is not disturbed by the IF amplifier.

Given LO noise of L = –160 dBc/Hz determined by using the passive mixer in receive mode, and conversion loss, L_c, of the mixer, the following can be derived for the transmitter. For an input IF signal level of +10 dBm, upconversion results in an RF signal level of +3 dBm at the mixer output with a noise floor of 3 – 160 = –157 dBm/Hz. When amplified by the 22 dB of external gain in the test setup (Fig. 6), the noise floor should rise to –135 dBm/Hz. The measurement setup in Fig. 6 confirms this level. Therefore, this one parameter of L (in dBc/Hz) can be used to determine the transmit noise floor.

REFERENCES

1. Mini-Circuits, Brooklyn, NY, www.minicircuits.com, frequency mixers, level 17.
2. H. Wohlmuth and W. Simburger, "A High IP3 RF Receiver Chip Set for Mobile Radio Base Stations up to 2 GHz," IEEE Journal of Solid-State Circuits, July 2001.
3. U. Karthaus, "High Dynamic Range SiGe Downconverter with Power Efficient 50-Ω IF Output Buffer," 2004 RFIC Symposium Digest, pp. 551-554.
4. K. Krishnamurthi and S. Jurgiel, "Specification and Measurement of Local Oscillator Noise in Basestation Mixer ICs," Microwave Journal, April 2003, pp. 96-104.
5. E. Ngompe, "Computing the LO Noise Requirements in a GSM Receiver," Applied Microwave and Wireless, July 1999, pp. 54-58.
6. Draft GSM 05.05 V8 1.0, European Telecommunications Standard Institute (ETSI), November 1999, p. 29.
7. MAXIM Integrated Products, Sunnyvale, CA, MAX9994/MAX9996 and MAX2039 data sheets, www.maxim-ic.com.
8. Requirements for spurious emissions in receiver bands, Section 6.6.2.1.4, ETSI TS 101 087 V8.5.0 (2000-11), p. 41.